Eventually your Sorcerer will be a major attack player. He is powerful in Elemental Magic and later Light/Dark Magic.

Early on the Sorcerer is good for finishing off enemies because with Static or Fire Bolt, etc he does not miss. (Flame Arrow and Magic Arrow do miss--- often and suck).
Skills are one part of two parts of promotion. He can increase his skill to expert level (Level 4 Skill), then needs to be recognized as an expert by Individuals who for $ will so recognize him. He then has to complete tasks that will elevate him to Wizard then Arch Wizard. With Arch Wizard Designation, and 12 Skill Points--- he becomes a Master and a deadly attack player. Staff or Knife for weapon. But it really isn't their go to.

Archers Paladin etc--- All have a similar path.

I usually designate one player a utility player. Repair, Open Chests. etc. But not every player can use every skill and you may want to designate one utility skill to separate players. But you only need one to identify items, one player to identify enemies, one to repair and one to capable of safely opening chests and boxes. (When one explodes, Perception will help any player avoid damage. No guarantee, but it helps.)

You have a Cleric which becomes Priest and High Priest. Heals etc. But an early use of the Spell Harm is a good back up.

Sorcerers and Clerics can offer protection spells. Useful against more powerful enemies. (Fire Archer is usually tough early on. Shield and Prot from Fire are good to have as example)

So get each player up to 12 Skill Points with Master Designation to max their major weapon/magic. Then add as you want. Skills will get incrementally better. But the major jump is to "Master" for each ability.

NPC can help a lot. Example: Scholar gives you boost in experience will identify everything for free.

1) "Best skills" depends on what you want each character to do. Look at the manual and you'll see what each character can learn. More on this below.

2) Skill points are effectively finite. You mostly get them from levels, with a few extras from horseshoes. Figure you'll be around level 70-100 by the time you're ready to finish the game. For the first ten levels or so, you'll get 5 skill points per level, and eventually the number of the skill points per level ticks up (so 11-25 might get you 6 per level, 26-40 might get you 7 per level, etc). Skill points per level listed here are only examples to illustrate my point; I don't know exactly which levels are the start of increased numbers of skill points.

3) You'll have to find teachers to rank up your skills. In MM6, anyone can master any skill they can learn (this isn't true for MM 7 or 8). You'll always have to get Expert training in a skill before you can get Master training. Generally Expert training requires 4 skill points in the skill. Master training requirements vary for different skills; Fire magic requires 12 skill points if I recall correctly, while Air only requires 4 skill points and a particular quest be completed. The trainer will tell you the requirements to receive training. Skill training (usually) requires gold, but gold generally isn't a problem by the time you're ready to get serious about reaching Expert and Master in your skills.

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-Only invest skill points in one or two melee weapons per character, and if a character is going to use a shield then that takes one of your weapon slots. For each character, look at their available options and decide what you want them to use long term. A two handed weapon will, of course, mean you invest in a single weapon and skip investing in Shield. I like to reach Master Bow with each character, and I only invest enough to reach Master.

-Try to split up who is using which kinds of weapons. If one person uses axe, second person uses sword, third uses mace, and fourth uses dagger, then you can use the best weapon you find of each type. If everyone uses daggers, then you're using the first, second, third, and fourth best daggers you find.

-For each character, only invest skill points in one kind of armor.

-For magic skills, decide who is doing what. Keep in mind that your cleric and sorcerer will generally have significantly more spell points available than your paladin and archer, so plan for your cleric and sorc to be primary casters. Invest enough skill points for your paladin to act as a back up healer to your cleric, and maybe have your archer invest enough to back up your sorcerer for any spells that remove status effects (like Stone to Flesh). You may want to split Light and Dark between your Cleric and Sorcerer, which each being the primary caster for one of those kinds of magic.

-Miscellaneous skills come in two varieties. Some affect only the skill's owner (Meditation boosts depend on each character's individual skill level), and some are effectively used for the whole party (only on character can open a given chest at a time, so there's no reason for more than one character to invest in Disarm Trap). For each "party" skill, pick who is going to invest in it; I'd suggest splitting these between your paladin and archer so that your cleric and sorcerer can focus more on magic. For individual misc skills, each character will need to invest as much as you see fit (I like to Master them and then leave them alone).

-Once you have an idea of how far you want each character to go in each skill, pick one or two of their skills as a "skill point dump" where all further points will be assigned for extra advancement. Generally this will be a single weapon or a single magic so you can increase your damage.

-I like to buy every skill possible for each character, even if I don't intend to use them much. Learning a skill only costs gold, not skill points.

Example skill mastery list:
Paladin
Weapon: Axe and one of Sword/Shield/Nothing - You'll mostly use an axe. You'll either use a Sword or Shield in your off-hand, or you'll use a 2 handed axe (skipping both Sword and Shield). Use a shield if your best axe is 1 handed (unless you decided to use a sword in your off-hand).
Armor: Plate
Magic: Each Self magic
Misc: Repair Item, Merchant, Learning, Body Building, Meditation
Dump: Axe

Archer
Weapon: Spear and one of sword/dagger - Spear in your main hand, and place a sword or dagger in your off-hand whenever your mastery level permits.
Armor: Chain
Magic: Each Elemental magic
Misc: Id Item, Perception, Disarm Trap, Learning, Body Building, Meditation
Dump: Spear or one kind of magic (to boost damage of a favorite spell; Air is popular for this, I believe)

Cleric
Weapon: Mace and Shield
Armor: Chain
Magic: Each Self magic, Light, and Dark
Misc: Learning, Body Building, Meditation
Dump: Light magic

Sorcerer
Weapon: Dagger, eventually dual wielding them
Armor: Leather
Magic: Each Elemental magic, Light, and Dark
Misc: Learning, Body Building, Meditation
Dump: Dark magic

So skills aren't infinite and I cant max all skills? Somewhere I read online that you can effectively get 999 levels. I also read that spear and sword can be a good combo for a paladin. Is that accurate?

Well there is no "practical limit" to skills acquisition but given that you need to increase your skill point total by n+1 to go from n to n+1 it starts to get harder to keep increasing your skill levels especially because the experience required to go up a level to gain skill points tends to increase geometrically.

As a strategy you should initially focus on putting 4 skill points into your desired skills and then attaining Expert proficiency in the skill for added benefit. You can check what these are when you right click the skill on the character HMI. Next goal would be to attain Mastery of a given skill and that varies across skill classes: weapons (8); armour (10); magic (12); miscellaneous (7). Although there are exceptions to this through completion of character class quests.

All changes to proficiency require finding the appropriate trainer to train to Expert or Master level. The majority of Expert trainers can be found in New Sorpigal or Free Haven.

If you want to wait around for long periods of game time and farm monster respawns, then sure, I guess you can reach level 999 (is that even the max?). It really isn't worth the effort unless you really, really enjoy periods of grinding followed by long periods of sleeping in an inn (or whatever). Reasonable clearing of a great many areas of the game will land you in the low 100's, maaaaaybe up to 150 (I've never bothered actually cleaning out the open maps of Sweet Water, Dragon Sand, Eel Infested Waters, and similar very end-game areas).

Considering that "maxing" a skill (assuming the max is 50) would require 1275 skill points per skill, maxing a skill also isn't worthwhile. There are enough points to Master every class skill that matters for a given character and still get another skill up in the high teens to mid-20's.

Spear and Sword is a fine combo; I mentioned it in the Archer merely out of habit (the archer starts with Spear skill). If you want to put Spear+Sword on both Paladin and on Archer (or swap the weapon list I gave for Paladin and Archer), go for it. I suggest splitting up weapons simply to give you more variety. No class has any special preferences or advantages (in MM6) with a given weapon as long as they can actually learn to use it.

isn't spear + dagger the superior combo?
anyways, focus on one style of magic per character for most part also goes for weapon skills (unless dual wielding).

Skill points are finite in a sense there are only limited numbers of enemies in the game. There used to be a bug where you can get unlimited xp, but it looks like that bug is fixed in the gog version. There is still a spot in one dungeon where it can spawn unlimited enemies, but it's a matter of grinding hard. Monsters on the world maps also respawn periodically. Some towns like New Sorpigal respawn every 6 months for example.

Your sorcerer and archer will share magic skills. I highly recommend splitting up the magic skills expertise, e.g. sorcerer does Fire/Water/Earth, archer does Air. Same with paladin and cleric with Spirit/Mind/Body. Archers and paladins are mixed classes that you need points on their melee skills too. Magic skills use up the most points (need 12 for mastery, except Air, Spirit, Light and Dark), so expect wishing your sorcerer/cleric to have more points.

In addition to melee and magic, you also need to do the utility skills (Disarm, Identify, Merchant, etc). If you have a knight, they are typically the one doing all the utility since they can't do magic, so they usually end up with most unused points later while your sorcerer needs to master everything. For mixed party like yours, I recommend splitting the utility skills among your Archer/Paladin.

You can also recruit followers to cover skills you can't use. For example, there is a follower that can do disarm if you don't have anyone in your party that can disarm traps.

Extra commentary on this point: If you're going to grind monsters, be advised that some maps and dungeons are 2 game years before a respawn. Extra commentary on this point: There are a few skills that can be covered by followers. I think there's one for ID Item (I might be mixing this one up with MM7), and a few different followers handle repairs for different kinds of broken equipment.

That said, there is no follower that can completely cover Disarm Trap for you; there are several that provide a bonus to the skill for each party member, but none of them guarantee 100% trap disarming.

The Scholar NPC provides 100% identify item skill and a 5% bonus to experience. Usually the first one that I recruit. This NPC is available in M&M 6 through 8 and also think 9 too.

Agreed on the Disarm Trap not having a 100% option with NPCs. One other option is the Telekinesis spell which allows you to open chests from a distance. What I have also found is that getting all characters to Expert perception leads to significantly less damage and death when a chest go's BOOM in your face.