Yes, i keep 7 juveniles in a 20g and have kept and bred a spawning pair in there before them (their parents).
Once adult i wouldnt recomend keeping more than 2 pairs in a 20 and they should have no tankmates, the females grow largest and can reach 6"+. The will require a large internal or external power filter and bi weekly 20% water changes as they are carnivors and gready eaters. Be sure to provide many rocks and pieces of bogwood for them to form territories.