There is no rule of the thumb here. I have seen a 12 inch MBU in with guppies and mollies and tetras. They are not that aggressive but you take your chances.
The Green puffer like those of similar size is hit and miss but in general if well fed not a problem.
The Fahaka however is a different ball game. Its just aggressive and can really be aggressive but again who knows?
Usually the larger the tank so you dont cram them in then it might be possible to house others but with a Fahaka I wouldnt.
So I think it is aggression and territorial but in the space sense not in a particular area if you see what i mean.