First question-- look at the anal fin, which id the last fin before the tail on the bottom, of the platy ( or molly, guppie, swordtails, most live bearers ). If it sticks straight back it is a male. If it fans out in a trianglular shape, it is a female.
Second Question-- the pearl gourami should be just fine in with your other fish.
Thrid question--gouramis, bettas, and guppies might all not be the best of friends. Bettas don't like things with long fancy tails and bright colors-like guppies--and they usually will attack them since they both are surface swimming fish. The gourami and the betta might be ok but there could be some problems with nipping on each other's fins.
BTW--please reword you're last question because it is confusing ???