Do NPCs match their XP with yours? I know they do the first time you recruit them, but if I were to drop one from the party and come back later, would they have any more XP than when I left?

Nope! Their XP freezes at the first moment you /talk/ to them, I think.

Also, I believe that their XP doesn't exactly match yours. What happens IIRC is that there are a few "versions" of each NPC at differing experience levels, and you'll get the version of the NPC closest to your own level.

Both of you are mostly correct. There are about 4 or 5 versions of each NPC, and which one you get is determined by how much EXP you have when you first recruit them, not just talk to them.