2 cories can work, just because they generally shoal, doesnt mean they cant be happy healthy fish with just two in there.
the basic rule of thumb for small tropical fish is an inch per gallon. were not talking oscars here. if you are dilligent with your water changes, you can always do a little more than the "rule" its a basic guideline.
IMO you can do 3-4 cories. just stay on top of water changes
ive seen bettas eat shrimp