Fish like mollies, guppies, leopardfish and platys all belong to the family Poeciliidae. Mollies and guppies are both of the genus Poecilia, whereas the leopardfish is Phalloceros and the platies and swords are Xiphophorus.
Some of the species are capable of interbreeding. The males of this family are well known to try and jump anything that moves - male or female! :-?
Many find keeping mollies separate is better as they are larger and get the food first. They also like a bit of salt in their water (1Tbs per 10L). In fact, if the salinity is slowly increased, mollies are capable of living and breeding in sea water with a density of 1.024 - 1.028, which is the same as the coral sea.