Gotcha! Well then scratch that, but you could always dabble in a new shrimp. Rare ones like blue tiger sell at 225+ for just ten shrimp X.X
I'm planning 6 in a 5gallon... I'd say if you're just trying to breed, you'll need an individual 5 or 10 tank just to breed one pair.
You'd keep them all in the same tank to start, but when they're ready to breed, move the male and female to the new tank, let them lay their eggs and then move them back so you can raise the fry.
SO it isn't all about the number you put in a 20.
In a 20 though, I'd say you could easily fit a few schools at 8-12 each..
The buggers get to .7 inches and have such a small bio load.