+1It's not very likely that an Angel will eat the Cardinals unless they are very small. Angels will eat Neons but usually Cardinals are ok once they get a little bigger. I'd be more worried about overstocking the tank. A 20gal is really only large enough for one Angel on it's own IMO. Maybe with a few Corys down the bottom. Having more Cardinals in the tank is no problem without the Angel. You might be better served by something like a Honey Gourami as a centre piece fish.