if the hf is very agressive you could try a rd but then you might get some very large fights! same with gt's.
if it is a little more clam it may also be housed with a couple jacks. these guys IME are never to aggresive but are aggresive enough to hold their own!
if it still has some growing to do jacks would be more likely to work if they all start out round the same size.
but if it is aggresive and you do not want to go through all the fights that are likely to occur with rd's and gt's you might try getting it a mate!