This comes up for discussion at regular intervals. The general consensus seems to be that guppies and mollies technically ought to be able to interbreed as they are the same genus; however, this isn't something that seems to be happening in people's tanks. Platies cannot interbreed with either mollies or guppies- not closely related enough. On the other hand, platies and swordtails can interbreed and frequently do so: a lot of the platies you see in the shops have some swordtails in them and vice versa. Guppies and endlers will also interbreed if given half a chance; and as endlers are a threatened species, one should avoid keeping them together for this reason.
