I bought 30 feeder guppies for my 10 gal. Some of them died within the first week, but the ones that lived started to multiply. They take a while to grow out into anything worth while though. You might want to get another tank to grow the the babies out.
I wouldnt do mollies personally, they get too big except the balloons and those have weak genetics. Personally, I'd go for 2 - 3 pre-hit female guppies and one male. Id get the fancy type because the feeder guppies are much smaller and their fry are consequently smaller and less numerous. Feeder guppies are often diseased as well and die on you. With the guppies you could even make a makeshift tank divider and throw fry on the other side of it when they are born so the parents dont eat them and they'll be easy to get at when you want to feed. Platies work also, but their fry are smaller than guppy fry.
Guppy sexing? This is probably the first animal you learn to sex in the fish world. Males are considerably smaller and have stronger colouration with much bigger tails. Females are bulkier, lack the strong colouration and have smaller tails. But females grow bigger.
Mollies would be the worst feeder's as although they have large fry, they have small broods. Guppies would have to have the smallest fry, but have been known to spew out 100 fry at a time. The most I've had is 89 with a 2.25" female.
Get a bunch of guppies, or mollies. the fry are so tiny. My wrestling halfbeak goes crazy whenever they breed. Gobbles them all up before I get a chance to move them.