1. Is it possible to farm experience from infinitely respawning random enemies, or are spawns limited?

2. Are any particular classes mandatory for any of the games? (For example, if the party doesn't contain a thief (for example), will the party get stuck at any point?)

3. Can you restart the game with the same characters without losing experience?

1. There are some spots with random respawning patrols in some of the games that you can farm.
I don't remember if they are in all of the games.
Farming for experience never occurred to me back in the day when I played them so I don't know if it's viable.

2. Not that I remember in the ones I played. Though if you don't have certain classes you can miss out on content. For example if you don't have a Knight in Champions of Krynn (I'm waiting fot the Dragonlance ones to come here too. :) ) you can't do some quests.
So the mechanic is there but I don't remember how much it's used.

3. Not that I remember. Once you've done a game with a party you can't play through the game with it again. But this I'm not certain of.

Sorry for not being quite clear. It was a long time since I played them. :/

1) Yes, you can farm areas for experience (for example any game with a wilderness area). Racial limits and low level caps in the early games makes this not a very viable strategy though.

2) Main quests shouldn't be a problem. There may be some sidequests that require certain NPCs or specific classes. I also haven't played these thoroughly in years, so my memory's a little hazy on this one.

3) It may be possible, but not likely to be much fun. The modules are designed so that you're pretty close to the level caps by the time you reach the end, especially if you explore a lot.

Flynn

1: Yes. And it's unlikely to be required to reach the level cap of a game. These games are not jRPG xp grinders, for some of the games you're likely to hit the xp/level cap before reaching the final area if you do all available side missions. Without fighting any random encounters.

2: Almost all of them get a lot easier if you have a decently balanced party, but it should be possible to complete any of them with any party. Some options are just going to be a lot tougher than others.

3: All of the games allow you to export a character from the party and then import that character to a new party. Those two parties need not be at the same place in the story.

Regarding #2, I've only seen a few cases where specific party composition matters:

* Having a Thief in the party during the Textile House mission in Pool of Radiance opens an additional solution option.
* Having a female character in the party during the Hap caverns in Curse of the Azure Bonds unlocks a mini sidequest.
* A couple of mandatory secret doors in Secret can only be properly seen by a Dwarf. Otherwise you have to find them through blindly walking into walls or by consulting a map.
* There are some trapped loot caches in Secret that are destroyed if the trap is not successfully disarmed (therefore obtaining the loot from said caches pretty much mandates having a Thief in the party).
* The Fighter NPC that permanently joins your party at the end of the first town in Treasures is dependent on the gender of the specific PC who is in the first slot when you first enter the "dungeon" area.
* One of the side missions in Champions of Krynn is impossible unless there is a Knight in the party.
* In Gateway, and in the Krynn games, there are treasure rooms that are more or less impossible to loot unless you have a Mage in the party who knows the Knock spell; they are Wizard Locked and even very high STR characters don't reliably open them.

In Pools of Darkness you need a thief to finish the extra challenge at the end.

There's a lever in Dark Queen of Krynn that can only be reached by a thief...in the last big area of the game, which can't be left once you've entered. So it's entirely possible to get that far and then be stuck. I had to create a thief and load her into the party into a saved game in that area and then take her out again.

That sounds like a design flaw. If the developers wanted to make the thief mandatory there, they should have made it impossible to enter the final dungeon without one. Alternatively, they should have made it optional; perhaps you miss out on treasure without a thief, or perhaps you have to go through a more dangerous area to proceed.