Yes convicts will work great too. My fish always have a constant supply of tasty snacks when the male convict isn't looking. I do usually end up with quite a few growouts though, and this time I have marbled ones!
You should put your guppies in at least a 20 long for the best results although a 5-10 gallon with good filtration, live plants (IMO a must for guppies to graze on and fry hide in) and plenty of water changes should do the trick. You could also do a big community tank of different livebearers and just keep your population down by using the fry as chow. Keep the temperature above 80 and feed a high quality diet with live foods and veggie matter and they will pump out fry pretty fast.
Speaking of guppies though, my baby sun cat decided to eat the last remaining females in my 29 so I think it's finally time to put him in the 125. That fat turd has grown pretty quick. I think my mollies will be next.