I had a 300gallon tank, and I kept a Arowana and a fahaka with no problems, for years.... but that is really rare. Mbu's are more docile, and would probably be fine. But you could literally only have the arowanna and mbu in the tank, anything else might crowd the mbu... If you got a few smaller puffers, they would probably still pick on your Aro. However mbu's are known to be fin nippers... and Aros have ver venerable fins...