I've also read that they don't have that 'tooth' like others do. Mine sucks snails out of shells. I've never, ever seen him eat a shell. I find the bleached left over shells all over the tank. Lol.
I'm not positive about why they need the 5 gallons but I can kinda guess. I don't know if he'd feel as comfortable with less space to call his own. He runs to a back corner when he's spooked and he does it FAST. Way faster then anyone I have. And at that speed he'll bonk the living heck out of himself on the tank walls. With less space, I think he'd completely freak out and he might do himself harm. But honestly, that's just based on my puffer and his persona. They might not all be skittish like that when you get close. I mean, he knows me and he still won't let me put my face closer then three feet without taking off. Hehe.
Everyone should have one! They're so wonderful! :D