I'm going to be perfectly honest, from my experience, I don't recommend wanting them or allowing them to breed. If you have one female, I would separate her. At first its a wondrous thing to see the babies and all, but live bearers breed so fast that before long you will have an overstocked tank and won't know what to do with them all. Believe me, I've have somewhere between 50-75 swordtails in a 55 gallon tank and couldn't find a store that wants em. Finally my brother in law said he wanted some so I gave him all my males and now all I have is females that are diminishing gradually through course of nature.
I just want people to know what they are getting into when they start keeping both sexes of live bearers. Unless you have a store that will take them, don't do it.