I just wanted to add to the conversation here, as I have a group of 6 corys in a 42 gallon that used to house my betta as well. They all got along very well, indeed. I started with 4 corys and after a few days they would hide and I would never get to see them play... I added 2 more and that seemed to make them happier and now I see them all the time. The challenge that I had was feeding. Everytime I dropped them shrimp pellets or wafer pieces, my betta would also eat them. So he was eating his meal, and taking from them, and getting bloated at least a few days a week. I removed the betta since they started spawning and I found my betta eating the eggs. He shares a divided 10 gal now. My cories have been spawning every other day for almost 2 weeks. I think that they would be happier in a group and you will need to watch feeding times, I was never really sure that they were getting anything to eat before the betta was removed....
Melman, black and white CT,
Fred, Red veil tail<-- started my addiction
Fish Fish, Cellophane Delta
Shimmer, CT maroon and teal with sparkles