Puffers can make friends
I set up a 10 gal tank a few months ago with just two spotted puffers. Well, I've been feeding them earthworms, frozen blood worms, brine shrimp and guppies. Well, each batch of guppies I put in, they'll leave 2 or so alone. So I have 6 growing guppies mixed in with these two puffers and no fighting or nipping. They are were small when they entered the tank, and now three are full sized.
I even let them go a few days with out food, and they still didn't attack the remaining guppies. Then when I fed the guppies flake, the puffers took notice and that's how I got them to start eating flake. I want to feed them guppies again in a few weeks, but if they leave any to live, the tank is going to get crowded.