I don't think you are understanding how this works.
This is a variable answer depending on size of catfish, size of tilapia, season (breeding season for the catfish vs winter), water temp not so much now as you have said mid 70's, but this will change with the season. It depends on how you feed, what you feed, frequency of feeding...sinking, floating, etc. By mixing species you intend to feed on each other, you are creating a food chain. You need to research them independently first. Gain experience with both, then mix and adjust to your specific style and location. How YOU do it, is not the same as a guy down the road, next state over, or across the country.
And that's BEFORE we start talking about maximizing production....egg stripping, egg tumblers, etc.

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.