I don't think even the smallest of screens will 100% keep fish from finding a way. Especially crappie. I think you can still have a successful catfish "only" pond though, especially if you keep it devoid of vegetation and ladder stock your cats. And remove them when caught. If you take this on with knowing you'll get some overflow fish, and be ok with that, should be happy with the results. May eventually have to get more predators to help out.

What were you planning on feeding your cats?