I don't think you will be overstocked (not until your mollies and guppies start popping out babies!), but if you don't get a "pair" of dwarf gourami's, and end up with two males or females, there is always the possibility of aggression between the two fish. In a 55 there is plenty of space for them each to claim a territory of their own, but sometimes, during feeding etc. there is the chance of aggression.
You know, in thinking about this, for a centerpiece type fish, you could also go with three pineapple swordtails (2f/1m), for wonderful personality and color. Once they are grown, they get to be 4-5 inches each, and will do great in a tank with other live bearers, but that is just my thought on it..