I gave a call to my nearest commercial fishery (90 minutes away...) and he extolled the benefits of catfish and 'shellcracker.' Seems shellcracker is a type of sunfish, like blue gill, but they grow faster and larger. They're called shellcracker because they like to munch on aquatic snails, cracking the shells in their powerful back jaws.
Most of what I can find on shellcracker (or redear sunfish) is on sport fishing or pond culture websites. I can find all kinds of recommendations for stocking density for a 1-2 acre pond, for example. And much of that literature explains the mix of fishes you want to get. Carolina Fish Hatchery goes so far as to offer pond stocking packages. For example, the package for a new 1 acre pond would run $765 and consists of:
700 Bluegill (2-4")
300 Shellcracker (2-4")
100 Largemouth Bass (4-6")
100 Channel Catfish (6-8")
I told the nice man at the local fishery how excited I was to visit once I'd set up my system. He proceeded to tell me about the Fish Wagon. Fish Wagon is a family owned pond stocking business out of Arkansas that delivers to customers at local agricultural supply stores during the warmer months. I'm sure the nice man at the fishery 90 minutes from my house wouldn't mind my business, but he thought local delivery of fish that didn't cost me shipping might work better for me.
I thought that was right nice of him.
If I'm looking at a polyculture of fish, I don't mind going for a 300 gallon tank. Even though I've read on the internet somewhere that you can have as many as 3 bluefish per gallon of water (900 blue gill), I think I'd settle for a tiny fraction of the numbers cited above.
Here's the way the Rubbermaid 300 gallon fish tank and 50 (or 100) gallon grow beds could fit in the 8'x18' space:
In other news, my tilapia have yet to ship. They're coming from near Kansas City, which has been hit by a big winter storm. Looking forward to having them in hand sometime next week!