I have the exact situation happens at this moment, that is a 1,5" baby dat with 2 of my greedy longfinned albino sens, 4" each, in a 40 g tank. Its been two weeks and everything seems to be OK though, but I will not try it with endli or ornate instead.
20 gall is way to small for your bichir at that size. You should upgrade your tank soon, otherwise you will end up stunting your bichir and they can grow properly.
Check out the polypterus forum for more information on bichirs.
cheers