I'm an obsessive freak, but I tend to play the campaigns using a single pokemon (barring the mandatory HM slaves). More specifically, I use my starter until the point where I can trade, then use my other DS (shut up) and my other copy of the game (shut up!) to trade over a fresh Ralts I bred. I've done that since Emerald, because I have a disturbing need to use Gardevoir. With the advent of Gen VI and Super Training, I now usually EV train said level 1 Ralts before trading it over to the fresh game.
It works for me.
No judging, Gardevoir's one of my favourites too. If I had a 2nd DS, I'd have probably done the same, swapping my starter for a Ralts. Would definitely make the game harder, considering Ralts and Kirlia don't learn many good offensive moves until much later and their base stats are really bad until evolution hits them like a ton of really sexy bricks. On that subject of Gardevoir though, one of my biggest gripes with XY was they locked my Gardevoir's mega evolution behind the Elite Four.
Feckin' Kalos Champion whatever her name was stealin' muh pokeman's mega stone...
