Rift Lake Filtration

With the requirements/parameters you want, it's hard not to recommend a small canister filter. It will provide superior biological filtration over the HOB's you list, with lower GPH flow and water always exiting below the water line (less turbulence). I don't see why a Fluval 104 or 204 would not fit the bill (although at twice the cost).

If funds limit you to the HOB variety, the Emporer 280 would be the way to go. The spray-bar driven Bio-Wheel would, IMO, offer better biofiltration. And, because of this feature, you can lower the overall flowrate without impeding on the Bio-Wheel function. The Penguin series biowheels are exhaust flow driven (and in my experience, unreliable, but that's an argument I don't want to start here).

Either way, you are correct in wanting to use a sponge prefilter with sand. I learned this the hard way. I'm sure others can offer details as to how to add this to your setup.

The rule of thumb that says 1" of fish per gallon of water is only even approximately accurate with very small fish (say less than 2" in size). The bioload a fish creates increases as a function of its internal volume (because width and height also increase), thus the 5 fish you are looking at would produce as much waste as a LOT of 1" fish. The five cichlids you are proposing for your 20 would thus seriously overload the system, as well as (probably) kill each other over territorial disputes. Rift Lake tanks are also especially sensitive to ammonia problems, because of the increased toxicity of that substance at higher pH levels.

If you are determined to have Rift Lake cichlids in that tank, however, you could have a very interesting community of small shell dwellers. Neolamprologus brevis, multifasciatus, and ocellatus would all be great choices. You could start with a few (note: pick one species only), and, if well-cared for, they would be likely to begin breeding and give you a very interesting colony to watch. You can get escargot shells at most delicatessens that will work perfectly for these fish, and will only cost you about $10.00 for 30 or so shells.

As far as filtration, I'd stick with HOBs, but add an extra one for safety. An Emperor 280 would be fine (the current would not be too strong), but then I'd also use an Aquaclear 200 or 300 loaded with nothing but sponges, for additional mechanical and biological filtration.