I have never used a multi-class character, and I wanted to ask if someone could tell me the pros and cons of playing one, or a party of them for that matter...?

Great! Thanks so much!

I'm glad it helps. :)

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While this is true for spellcasters, it's not true for fighter-types like the Fighter or Barbarian. Attack bonuses stack when multiclassing, so a Fighter/Barbarian multiclass is just as strong in combat (arguably stronger, actually) than he would be as a single-class character. The Fighter, Paladin, Ranger, Barbarian, and Rogue are all very well suited to muticlassing and lose very little for doing so.

Read what you quoted again. A fighter/barbarian is focusing in two classes (something I acknowledged as fine for non-spell casters), and will be much more effective in his chosen role than a Fighter/Barbarian/Paladin/Rogue/Ranger.

A FBPRR will have mostly full BAB, but they won't have any of the higher level abilities of any of those classes. No skill points or sneak attack from the rogue, very few bonus fighter feats, only the most basic barbarian rage, next to none of the ranger special abilities, and not even divine casting from the paladin (though I gather most of the paladin special abilities are in the first few levels, so there is less lost here). Their saves, except maybe fortitude, are also going to suck because they won't advance in any class far enough to get many of those off-class save increases.

That would only be a problem if the high-level abilities were stronger than the low-level abilities, but by and large this is not the case. In fact it's quite the opposite, some of the best abilities appear at very low levels, making it easy to shop around with multiclassing. Unlike spellcasters, their abilities don't go obsolete at higher levels. A 1st level barbarian rage will still boost your attacking power even if you're 20th level, while a 1st level Magic Missile is a joke. IWD2 has a very low number of skills in total, and doesn't even require you to max them out to pass all the checks. A Rogues 8 skill points per level is complete overkill.

Sneak attack is terrible in IWD2. You can only ever activate it once against any given enemy, so its effect is easily dwarfed by smaller values that apply on all attacks. Even with the correct rules implementation (as seen in such games as Neverwinter Nights) putting all your eggs into the sneak attack basket is unwise since so many creatures are immune to it, and diversification is almost always preferable.

There's only so many good combat feats, and fighters run out of great picks relatively early. There are very few reasons to remain a single-class fighter, and this is one of the few classes that is virtually always improved by multiclassing.

Basic barbarian rage is absolutely amazing and one of the best ways to boost your damage output. I didn't say to take all the classes. But if you did, Paladin's divine grace and barbarian rage both give will bonuses to saving throws that will far exceed whatever you lose from multiclassing.

Let me ask this: what is your recommendation for leveling a character? How many classes should/should not be mixed in on a particular character?

Depends on the character. Generally you'll have one class as your primary, and then keep your other classes low.

Paladin is basically all about the Divine Grace class feature, which is better than all his other class features combined. You can always take the 2nd level if you like aura of courage, but there's not much reason to take more than 2 levels of Paladin. Any character who can afford a decent charisma score and is lawful good will benefit enormously from a level of Paladin.

Rangers are a very specialized class, and only fit in stealth-focused parties. If you aren't focusing your entire party's strategy around stealth, there's not much point in having a Ranger. His entire niche is being a dual-wield warrior who wears light armor. If you're doing that, one level of Ranger is an easy way to pick up both the feats necessary for dual-wielding. If you're not doing that, there is zero reason to take this class. They're completely outclassed by a Fighter/Druid multiclass characters (1 level fighter, rest to druid) as nature-themed warriors.

Barbarian is also applicable to many builds. Rage is a very nice way to boost your offenses while also giving you better will defense which is the big weakness of fighter-types) against mages, but more importantly it has faster move speed. This allows you to use ranged weapons to play hit-and-run games against enemies. Thrown weapons or bows both work well for barbarians, and strange as it may sound it's actually one of the best ranged-attacking classes as a result. Uncanny Dodge is a pretty decent ability and worth taking a 2nd level of barbarian, but unless you plan on taking it to very high levels there's no point in going further than that.

The Fighter is pretty good for 2-4 levels, but the fact that you'll pick up all the good feats relatively early and run out of decent selections means he gets less and less attractive the higher level you are. As a result, single-classing as a fighter is generally a bad idea and you should always multiclass out into something else. Usually 4 levels of Fighter is the most you should take. Due to his better value at lower levels, two levels of Fighter is easy to fit into almost any build.

Next we have the Rogue. The Rogue is kinda underpowered in IWD2 due to its nerf to the 3rd edition sneak attack rules. However, the Rogue is still good for skills. Multiclassing Rogue for just one or two levels can be all you need to cover one additional skill that none of your other party members can. The Rogue also has the evasion ability at 2nd level, which is a great addition to any character. That makes two levels of Rogue the ideal.

Monks can be an interesting choice for multiclassing, although his conflict with wearing armor and his weird multiclass restrictions make that hard to pull off. Generally the best approach to multiclassing Monk is to begin your career as a Fighter then enter the Monk class later, or to take 4 levels of Monk for its superior saving throws, Still Mind and Evasion, then abandon the class afterwards.

Wizards and Sorcerers are not very multiclass friendly, although Sorcerers can take one level of Paladin just for divine grace. Bards, Clerics, and Druids can all take one level of Fighter (or another similar class) for weapon proficiency, but shouldn't take more than one level out of heir primary caster class.