At your current experience level, buy 2-3 and learn with them. 600 gallons is NOT gonna support a lot, or very big. And that's IF you just had cats in them. Add the Tilapia, and you need a hell of a lot more water. You pack in the density, and disease will break out and wipe both out quickly.
I was planning on stocking the tilapia first, about 50. Then as they grew and started breeding I would add a couple catfish and see what happens to the population. Is the stocking of 60 fish in a 600 gallon tank to ambitious to start? I was thinking that would be on the light side (which is what I want so like you said problems wont be magnified).

Thanks bsfman, I have a similar setup as you with the pump in the fish tank. I notice it already sucks up few small minnows I put in my system so maybe I wont need a predator after all. I guess I just need to wait and learn my system as I go.

I really appreciate the advice, gives me things to think about before I get there.