Yea, if you guys were to split an order you could do well. I highly recommend Channel Catfish. We think they are great eating and in my experience here in Florida with a non heated system (not in a real greenhouse) the catfish will grow faster than mixed gender tilapia any day. In a 600 gallon tank, the catfish will grow nice and big.
On the subject of bottom dwelling and the catfish getting food. My catfish get floating feed and they come to the surface for it. It will be interesting to hear how they co-habitate for you. Once the catfish get comfortable in the new home, I expect they will be bold enough to get their share of food. If you are worried that the tilapia are bolder and getting all the food, I would suggest adding a feeding pre-dawn or after dark or both. I find that my catfish feed better at dark or twilight and are not very hungry mid afternoon while the tilapia arn't nocturnal and will sleep in the dark.