Ok- first of all, which kind of red eye puffer? There are three different ones that go by that common name: C. Irrubesco, C. Lorteti and C. Salivator.
I currently keep a C. Lorteti, so I will tell you about him.
He was originally kept by my friend in a heavily planted 30 gallon tank with 2 females. He pretty quickly decided he did not want them in his space and killed both of them. Now he lives in a 10G by himself, and I really wouldn't feel comfortable keeping him in anything less. When my friend kept him he tried a variety of tank mates (cories, plecos, otos) and none of them escaped his wrath. These guys are very mean, and very intolerant.
That being said, I love him, and he is the smartest fish I have ever kept. He's a total attention lover and always swims to the front of his tank to beg for food, which he will eat right out of my hand (although, be careful, cuz they have *sharp* teeth). He eats raw shrimp, any snails he can find, and any other live food I give him. I haven't heard of any teeth problems with these guys, but just in case, it's always good to feed a steady diet of snails.