I have had crappie in my pond for years and they have never had a successful spawn as far as I know. I also had a few 20# channel cats and they eat ANYTHING, flatheads would be even worse as about all they will eat is live fish. I personally would never stock any catfish besides bullhead in a pond. I do know channel cats are popular and can be managed fairly easily. So if you really want to do it, go for it but I don't think your crappies could support the cats for long without sunfish or shad in the pond as well.