Hi l.michelle
I just saw your post in the Betta section that directed me to this thread. Corys are very entertaining fish and they usually do well with a betta in the tank.
I've noticed that some bettas will become more active and interested in their surroundings with corys. In fact I had one called Ruby that thought he was a cory. I would even find him squeezed into their cave with them. He would follow them all around and watch everything they did with much concentration. The corys, of course, ignored him. IMHO, this is so much better for bettas than just having a boring and lonely existence by themselves.
There are three species of corys that are very tiny. They are C. pygmaeus, C. hastatus and C. habrosus. While they are fine to keep, they are not nearly as interesting as the full sized corys. A ten gallon tank could house many of the tiny ones, or up to 4 or 5 full sizes corys with a betta. This would assume that it's completely cycled and that you do weekly water changes, of course.
Corys like caves and they make good tank decorations. All you need is three rocks and a flat piece of slate to go on top. Plants are good too, especially floating ones so they can break up the water current from the filter and make it more comfortable for the betta.
A heater is a good addition since it will keep the temperature steady and you can keep the tank at a comfortable temperature for them. What temperature you keep it at will depend on the kind of corys you get because some species prefer warmer water and others cooler.
Bronze or albino C. aeneus are the species I usually recommend to beginning cory keepers. They are easy to find, have great personalities, and can do well in temperatures between 70 and 78 degrees F.
Please let us all know if you decide to get them.