Any of the neocardina species would work. Plus, they breed like crazy. I started with 10 Cherry Red shrimp last October in my 15 gal. I just sold 100 of them, and it did not even make a dent in the population!
The problem with dwarf corys is that they prefer groups, 6-10 minimum. You might be able to house 6 c. hastatus or hasbrosus if you have plants. I still think that tank is too small, though.