I can say from experience that guppies would be better. I've had 20+ fry per mature female, so if you have a 30g you could start out with 3 males and 6 females and you will have fry in a very short period of time!
@Heyguy: I thought that convicts protected their young, so wouldnt that entail that they dont eat their babies???
10 gallon: 1 betta, 5 neon tetras, 1 mystery snail
29 gallon: 1 gold & 1 opaline gauramis, 5 serpea tetras, 5 pristella tetras, 4 neon tetras, 3 cory cats, 1 ghost shrimp
75 gallon: 8 electric yellow labs, 7 cobalt blue zebras, 4 acei, 2 OB Peacocks, 1 rusty(?), 1 chinese algea eater, 1 albino bristle nose pleco