Channel cats respond well to a pond environment. They do extremely well when fed - just look at the catfish industry in the delta. Blues may do as well, but I doubt it or we'd see farm raised blue cats on menus.
Flatheads are apex predators.
They're going to need to eat something fishy. If you have nothing else in the pond but channel cats they'll eat them. Pretty soon you'd end up with a pond with one or two BIG flatheads and very little else. Channel cats respond well to a pond environment. They do extremely well when fed - just look at the catfish industry in the delta.