it depends on what level swimmer they are. you could easily get a nice school of cories and then add some mid or top level swimmers. what else are you interested in?
I had 6 cory cats in my 20 gallon with a pair of pearl gouramis and a black molly. (They are in a bigger tank now with other fish.)
If you went with smaller tankmates, you could still have 6 cories with a higher number of tankmates. I have some little black neon tetras that are pretty and go well with the cories.
Corys need to be in groups of at least 4 to be happy, albino, peppered and bronze corys are good starter ones as they are quite hardy; what other fish do you have in your tank?