Couldn't agree more : a) tank too small, for the labs alone b) tank wrong shape (want long tank) c) labs and Angel = dead Angel.
If you want africans in a 20, consider ocellatus or similar shell dwelling cichlids from lake tanaganyika. www.cichlidforum.com ; some suggested mixes here.
I guess you could try leilupi, but I don't know that a 20 high is enough floor space for 4. Perhaps 1 male, 1 female.
I think you'd probably get away with a pair of rams, seeing as you ask, although a 20 long would be much, much more suitable.
Have you ever tried special ordering fish? If you're young you can have your parents do it for you. Hit them up with "It'll increase my interest in science(Well, it does)! Yadda yadda yadda"
I was thinking more along the lines of buying four, so you can get a pair. Many juvinile Tanganyikans are hard to sex when they are little. This is my nearly surefire way of getting one of each. (though, I have seen this method get someone four males!)