He is definitly not telling you the complete truth.
Puffers can range in social behavior. The MBUs I have kep were hand fed and always kept with tank mates. Never had a problem with anything that didnt try to bully him, or that was a invertabrate. Fahakas are a different story as they seem to kill things in the tank just for fun.
I have heard that MBUs get aggressive towards each other when they get bigger, and never wanted to risk one of them to test the theory.
My Mbus have always been little diggers and like to dig little ditches to sleep in.
I would recomend adding some sort of substrate. Their waste is big enough to pick up as it is.
Those are gonna have to be some really big tanks.
I would think 180-200 is pushing it for a single adult MBU, let alone 5. Would deffinitly be a site to see. 5 30" puffers in a single tank.
That would be like heaven.