Yeah, no fretting. Mollies, Platies and guppies will have babies even in the
LFS. Your conditions will be a lot better than theirs (I assume).
Two other comments.
1) New-born fry are safest when your planted tank has dense vegetation with small leaves or filaments. The most highly recommended plant for birthing is the Java Moss, as many non-libearers tend to choose it to lay their eggs in, and as livebearer fry tend to hide in it for protection. If you can get any aquarium-safe moss, get it. Your
LFS may not stock it. (This isn't imperitive.)
2) If you're really concerned with the safety of the fry, you might want to look into getting a breeder enclosure or a separate tank. I've got an Aqua Nursery from Penn Plax. It isolates the pregnant livebearer. When she gives birth, the fry are gently sucked down a tube by the air pressure caused by an air pump, and into a second compartment, where they are safe.
http://www.bigalsonline.com/cgi-bin/view.cgi The picture is a little hard to make out... But the mother stays in the chamber to the left, and the babies move down the tube at the bottom to the chamber at the right.
Choose one or the other option, and return the babies to the tank when you think they're big enough to brave it on their own. If you can't get to her before she has the babies, try syphoning the babies out of the tank gently, and moving them to another, safer tank.
Do you have any fry food? Liquifry or First Bites (Hikari) are what I use, and my fish grew well. Before that I'd been feeding them only algae wafers and crushed flake food. They weren't enjoying it.
Good luck!