The incandescent bulb would probably keep the hornwort alive in a 10g, and you would also probably end up with a lot of algae due to (likely) improper spectrum. Ime, for livebearers, I always just fed the fry ground up flakes and other freeze-dried foods and they were fine. The fry are fairly big by the time the mother gives birth. I think they'd be large enough to eat baby brine shrimp (frozen or whatever) from the get-go.
As far as I know, you will see a dark spot near the female's vent, you may even see little fry eyes when she is ready to give birth. Are the males following her closely?
About the light, if it were me, I would pick up a 15w 6500k spiral CFL at walmart that fits your desk lamp. It would grow the hornwort much better and, to me, would keep the tank much healthier. Hornwort is a great oxygenator, but if it isn't growing well, it isn't benefiting very much. I think GE makes them.