Agree with mambee. Bigger is always better, IMO
. Whether they go with kuhlis or corys, a group of 5 or more would be best. Kuhlis aren't quite schooling fish, but they prefer the company of their own kind. Corys, which are schooling fish, definitely prefer larger groups than 2.
I think either would do fine with a betta. Kuhlis rarely stray from the tank bottom, except after lights out. Corys will make frequent trips to the water surface for a gulp of air, and may explore all areas of the tank periodically, but are quick enough that the betta, if it's inclined, won't be able to harass them much.