Your pretty much limited to fry and juvenile growout for 20's. You might be able to breed a pair of the smallest cichlids (lanisticola or some sort of dwarf labidochromis) but it won't be ideal.
What about julies, a pair of brichardi, 3 or 4 yellow labs. I think there are a few more options wityh a 20 , especially a 20 breeder, than just shelldwellers