Level limits for non-human characters will become an issue starting in this game.
For each of your characters:
* Human Fighter: OK, but you might consider replacing them with a paladin or ranger.
* Elven F/MU/Thief: You will be able to reach the maximum mage level in this game. This should work well, although your fighter levels will be capped. (Do, however, note that this character won't get any more mage levels in the next game, so CotAB is the last game where this character will be good.)
* Half-elf F/Cl/MU: Replace this character. You won't be able to get any higher level spells, and the cap on cleric levels is way too low, to the point where you can't even get the second healing spell with this character (and the first one is absurdly weak).
* Human thief: OK, but note that, unlike other classes, non-human thieves can reach the same levels as human thieves. (Also, I've heard that thieves might not be that useful in this series.)
* Human wizard: OK, you can keep this character.
One thing I would recommend is creating a Human Cleric, or alternatively having a Human Fighter dual class into Cleric after reaching level 7 (but no later than level 9). This is the only way to get 4th and 5th level Cleric spells in this game.
One warning: Don't dual class right away, as the game will trap you in an area with no town access really early, and when you do get to train again, you can only gain 1 level, with excess XP being lost. In the meantime, the character is functionally a level 1 character with extra HP.
carbon_dragon: Note I'm new to PC games so I'm not sure I'm up to character editing my existing characters.
The way to do that is to use a hex editor, and look for the bytes corresponding to your stats. The file is not encrypted, and there's no checksum, so editing your characters is easy. (Do, however, note that the game may not behave as expected if you change values outside their normal parameters. I remember, in one of the games, setting a character's gender to "2" (displayed as Copper), which allowed them to get 100 Strength, which is not as good as it sounds.)
carbon_dragon: Presumably if I did create a new Ranger, it would start with some experience.
Yes, your new character would start with some XP, but not as much as an imported character would have. My understanding is that this only matters early, as you're going to reach the level caps anyway.