I am going to remain against popular opinion and say I think a betta and a small school of pygmy cories would be ok providing there are plenty of hiding spots and you do water changes as required. That's purely from a stocking viewpoint, I don't know how they would get on.
Why not shelve the dwarf cory idea for another tank? They do much better in a large group, ideally 10 plus, are very active and occupy more levels than just the bottom of the tank, and like plenty of room. I'd say 20 gallon minimum. Thinking of the bettas I've had in a ten gallon, not all of them would've been happy with ten corys swimming around them all the time.