I just put in whoever is good for what I need. You really don't stay in one area long enough for anyone to get too far ahead in levels. I have 1-2 pokemon that are consistently ahead, but if one really lags behind you can just throw an exp share on them and have them #1 in your party and switch them out right away. They'll grab tons of experience that way.

EDIT: as a note, I only really use 6 pokemon unless I absolutely need another one. when I say 1-2 get ahead, it is 1-2 out of the 6, but an exp share evens everything out.

i ususally find 5 or 6 good pokemon, and level them up pretty evenly. everything else goes into storage to rot lol.

I always try keep mine at the same level.

I tend to keep them all ruffly the same level and I do mean ALL. I like to catch as many as I can too, so as you can imagine this takes a extremly long time to get anywhere so if you are not into grindfest I would not suggest that. I would suggest keeping atleast a couple high level ones though haveing only one high level and rest low level is bound to make you hate the game. Plus so many different types of Pokemon and abilites you would be missing out on so much!

buy one of the two new ones, theres no real storyline so it doesn't matter which one you start with