Mollies are somewhat similar to swordtails. You should be able to see a dark area toward the back of the belly if the female is pregnant (with black mollies this can be difficult). I'm not sure what the exact gestation period is for mollies but you should be able to find it somewhere on the internet because they are a quite popular species.
I have a group of sailfin mollies in a tank. Along with the adults there are about 60 fry from 3 different broods, diamond killis, and 2 small white mullet. Everyone seems content and I have not seen any of the adult fish chasing or picking on the fry.
Good Luck with your babies