They ARE predatory fish, but that doesn't mean they are bullet proof

Its not exactly contradictory in my mind, typically the Puffer is the aggresser, but it seems that they either pick on or GET picked on. He may not be able to distinguish the Guppies as individuals, or be able to single one out. Its hard to say. They are delicate fish, but they are territorial and aggressive fish, and don't handle stress well.

I have kept both Dwarves and Green Spotted Puffers(inseperate tanks) with other fish, and I wouldn't call either instance "successful" although my GSP attempt was much LESS successful then when I kept Dwarves with other fish...
I'm not sure if its because they're so small, or because they tend to seem to live in groups that they seem a BIT more amicable then other Puffer species.

If you think your Puffer is getting stressed out and/or injured (even slightly), its best that he or she be kept alone or behind a divider.

HTH
Malli