My friend has a cory cat with her betta in a 2 gallon, since they can also breath air (I didn't find that out until I bought an aquarium book I bought). She keeps it very clean which is probably why the setup has worked. I wouldn't put anything in there unless your EXTEMELY religoius about water changes.
Who am I? Well I am a Junior Member of the International Betta Congress and a breeder of white HMPKs, red dragon HMPKs, black/red dragon HMPKs, metallic multi HMPKs, blue masked HMPKs, and now multi color HMs. Soon to add black HMs. I will be showing in New Breeder class in fall of 2012.