GT's really are more of a medium cichlid in my opinion. I have a nearly full grown male in a 55 and will be moving him to my 75 here soon once that tank is free. I wouldn't keep him with a severum in a 55 by any means but many people have successfully kept them solo in that size tank just keep up on the water changes. If you're like me you'll want to give him more space eventually. They grow really slow so you'll definitely have a while to decide if you really want tank mates but if tank mates are really what you want I wouldn't personally keep mine with any other fish in a 4 foot tank he is one of those GT's that lives up to the name and will bite my fingers. I tried keeping him with some large buenos aires tetras when I first got him as "target fish" because he was wild caught and shy and he murdered them all in one day.