Normally,I'd probably advise the Miniature Cories in a tank that small,but truthfully with a Betta,I'm not sure that I could trust them.You never know about Betta's.

Peppered(Paleatus Cory) get a decent size,but you could probably get away with 2 -possibly three if you had a decent filter,and you let the tank cycle before you add the Cories,and keep up on maintaining Params of the tank. I have some five gal. tanks that are shaped long,and I've kept a few cories in them before,and not had a problem.

Peppered Cories have been one of my hardier Cories. As for the Betta,each one is different.In general,mine have left the Cories alone.You definately would be maxed out though,and couldn't add anything else for sure.