I agree that ghost catfish are really neat, but I'm pretty sure that 10 gallons would be too small for them.
Beta novice: Not sure what you mean about "black ones not brightly colored ones", but angels and bettas do not generally mix. Unless we're thinking of different fish, I don't see how a different colour would change an angel fish's behaviour?
Don't get a pair of Cories. I would get, at the very minimum, 3. But as they're shoaling fish, the more the better! Most people recommend keeping around 5 or 6.