I picked up my Black Aro today He is about 14"-15" in my 210g tank. And don't worry my Gulpers won't eat him. He is to tall and long to fit. And they have had tank mates with no problems My smallest Gulper swims with him its kinda funny.
I also have a 9" Green Terror that is very friendly that's going in there once he heals all up (when I got him he was in a tank with puffers and his fins were a little bitten up so he is healing up in my other tank)