I was inspired by how you gave out levels in your RP ;) It's always a bit tricky, but having seen slls81 only give out 1 level at a time in his RP and how slowly the pokémon grew there, I think 2 levels is a good standard to give out, but since the starting pokémon's levels were pretty high I've only given out 1 level at a time now in the beginning areas.
The player can choose if they want to train a few pokémon or capture a full team and divide their efforts of course (like the difference between Ashton and romdinner's character in A Fresh Start ^^ ). If a battle was awesomely splendid, I might feel inclined to give out 3 levels, but then it should really be spectacular and probably pretty long. We'll see if that happens.
The number of pokémon used in a battle doesn't affect how many levels an individual pokémon will get, but if the battle becomes shorter or less detailed because the player wants to cram in all of their pokémon there but don't want to make a mile-long post, I might give fewer levels. That only means that the quality of the post (and battle) matters. I try to keep in mind that people write differently and can be on different skill levels when it comes to writing, and be fair in regard to that.
Hm, the part I got from you was perhaps mostly that if someone captures a pokémon, the pokémon that battled will only get 1 level. It's a choice, whether you want to train the ones you have or use a post capture even more. Ashton clearly made the latter choice xD But since you can make any number of posts in an area, you can still make many pokémon grow if you're prepared to make more posts.
All this is not set in stone though. Let me know if you think you've been treated poorly, but I'll probably not change my mind ;)