20 gallons should be good, I'd think - get lots of females and fewer males so you get the most drops. Could easily go smaller, since it doesn't sound like these are a staple food source for the cichlids.
Alternately, since the tank's also for enjoyment and not just food, if you get just males for them, you can stock pretty heavily with guppies, a number of people posting in the livebearer section have male-only tanks stocked a bit over one fish per gallon.