Although these fish don't get real large, I think a 20 might be a little small. Better quality yellow labs can get closer to 6" but the ones you see every day at the local aquarium shop typically fizzle out at 4". I'd do a 4' tank with lots of rock work and caves and such. THis is what they would prefer. It will also give them the room to have thier own "home" in the rock work and the ability to get away or retreat if needed. If your absolutly stuck with a 20, perhaps 3 to 6 little ones and thin them out as the mature to like 3 or 4.
I would suggest keeping all females or if you want to spawn them keep one male with 3 or more females. Two males might not get along to well. You want several female so the male can't overly harass them. Males often harrass and even kill the female if there is just one male and one female. Sexing these guys can be hard when they are little. Males often have more black in thier dorsil and anal fins but this is not a 100% method. When they get larger, the males anal fin should be more elogated than the females but again, this is not a 100% accurate way to sex them.
I don't like sand because it can be tough to clean with out sucking it out. Aragonite, such as Carib Sea's "seaflor" substrate is heavy enough to vacuum with out sucking it out and it will positivly buffer your water. It in most cases will help maintain a ph in the low 8's which is what these guys prefer. I say "in most cases" because if you have extreamly acid water, It won't buffer that high.
These guys are not picky eaters, I feed them a good mixed diet that include vegtable matter like spirulina flakes. Avoid overly meaty foods or single ingredient foods like brine shrimp for example. They can cause digestive tract problems, espcially in juvies.
These guys have a neat and contraversial history. Most of the ones found in the aquarium trade are supposed to be from a single pair that belonged to Pier Brichard in the early 80's or so. That's how the story gose anyway. Another interesting thing about them is that they are some what of an oddity in the wild. The most common ones are more of a silvery / blue color hence thier name Labidiochromis Caeruleus. Caeruleus means blue. I guess that has nothing to do with your question but interesting none the less.
Hope this was usefull, I'd really concider a bigger aquarium though...
Joel