Interesting nmonks!
I've wondered about this stuff as well. It seems like several people has c. irrubescos together, and it works well. I have a pair in very small space (is awaiting larger tanks - I will probably get them tomorrow

). I haven't had them for so long, but it seems like they get along quite well. I won't trust them in the long term, though. Puffers are puffers
What has puzzled me is that pet shops keeps puffers together in the same tank, which is even small. And it seems like it goes well! Where I bought my c. irrubescos, they had several in a 50 liter tank. They even sold them in pair (I don't think you could get only one. They were priced as pairs). Where I bought my leiurus/turgidus/whateverheis, they were two together i a small tank. Tiger is a big puffer (at least compared with my other puffers), and they were two in a small space. As far as I know, tiger is a rather violent puffer, but he didn't hurt his tank mate, even though they had to live very closely together.
My c. Salivator, though (the one that is still in the shop), had killed his wife. Those two was also to be sold as a pair, which I still don't know why. You see how that worked out! And the other Salivator pair in the same tank (which was bought by Taly:-)), did seem to get along in some way, but she had to split them in the end, even though they have plenty of room there.
Those c. lorteti at the shop, were living very close together. Many of them in a small space. And I couldn't see one puffer that was hurt in any way!
I can't get it: many puffers in small space: it works out in some strange way. But few in a big tank, means being mean to each other. To me it seems like if you squeeze together an awful lot of puffers in too small space (I would never do it, just to make that clear), they just have to get along. Maybe the situation is so confusing for them that they forget to be aggressive to their tank mates. Or they just see that they have to cooperate to get out of there alive. I just can't get it.