Don't put the babies in their own tank or nursery. Let some of them get eaten by adults. Even if it is your first batch, I would not do this, although the instinct is to want to save them. There will be plenty more. Even with adult livebearers, they will not eat all the babies unless you have significantly larger fish, but then you run the risk of adults being eaten. Giving them away to the LFS
is your best bet if they will take them.