I've had my pygmy cories in with my betta in a 10 gallon for 9 days now, and they really go great together. My betta was checking them out in the bag while getting them adjusted, and chased one for a few seconds went they first entered the tank. He quickly learned that they are way to fast for him, and ignores them for the most part now.
I got a small school of 6 and they have plenty of space/filtration. They are excellent cleaners, very social and playful. I highly recommend them, but make sure you have some soft substrate like sand/smooth gravel/soil. I would also recommend a filter, and letting the tank cycle before adding stock. Without a filter and/or alot of live plants, you're going to be doing a LOT of water changes.
Last edited by Gale; 10-14-2012 at 10:55 AM.