I really would recommend Pygmy Cories. Not only do they stay small, they are quite possibly, the cutest type of cories ever. :P I have a school of five of them, and they are very entertaining. The only bad thing about them is that they can be a little sensitive. But at long as the tank is cycled before hand (or at least had a jumpstart with the cycle) they're pretty easy to keep. They prefer sand, but small smooth gravel will work just fine.
Calypso - Female Dragon PK (5.5 gal) Locutus - Male HM (2.5 gal) Fluffy - Male CT (29 gal community) Kihoo -Male VT (2.5 gal) ----- RIP Feesh, Zephyr, Xavius and Kami.