Baby Mollies-- please help!!?

I went away to Europe for 2 weeks, and when I came back I found one little brown fish hiding in the s. I have 3 GloFish, 2 catfish, and 2 Mollies. The one Molly we know is a male, and the pet store told us they weren't sure about the other one. It appears to be female. We've kept Mollies for over 6 months now, and although we've seen them mate, no babies have appeared. Now I've moved the one little fry into a smaller tank, using water from the big one, and put a couple of pebbles and a plant in it. I'm still lost on what to do, though, and why this is the first time my Molly has given birth. Please only give information that you know is correct. Thanks for any help! (: