My roomate breeds multi's and the largest a male will get is about just under 2 in. And thats a big one. With all of these multi's in one tank you are going to have a problem when they pair off and start breeding. You could probably get away with two pairs in a 20long but my roomate just keeps a pair to a 10gal and that works out perfect. There are about 20+ fry in the tank right now. The male will hover over the shells and keep guard of any other fish that come around, it is very hard to keep a pair with other fish if they are breeding. I would wait and see how many pairs you can get out of them and then break them up into seperate tanks for breeding purposes, and be prepared to get rid of the ones that dont pair off for they will just be destroyed by the pairs. But once they breed man they dont stop, and the little guys are so cute. So best of luck to you and your new fish.