Man africans are smart, If you put diferent sized hiding places for different sized fish they can really fend for themselves.
I had (not anymore) a psuedotropheus crabro that was...4.5-5 inches in my 20 long.
And there was a 2 and a half inch red zebra and this blue one that was the color of the backround of this forum and he was maybe about an inch to 1.5 inches.
I kept them in there for about 3-4 months and of course got them all at different sizes, but there were never torn fins. Africans are much smarter than the american cichlids (i found this out when I moved them to my 75 with a jack dempsey who was as big as the bumblebee african).
I had the least problems with my african cichlids in that 20, its definitely possible as the long has more of a foot that the 20 high.
good luck, just don't leave them in there too long like i did.