Mixing sizes is no issue... makes things more realistic, as others have said.
but repainting ducks into geese may not work that well. I've noticed the overall shape of the decoy does play a role, especially for wary birds. scaup (bluebill) decoys painted to goldeneye (whistler) pattern just doesn't work. Same for (general) divers to puddlers or vice versa.
A few mixed in weirdo midget goose dekes in the spread may not really matter, but would likely be just as effective left with their original duck paint. they do flock together (more often in field than water)