From what I've read (never kept puffers myself, but I've been tempted) putting different species together would be a bad idea. even members of the same species will sometimes bite each other; different species is just asking for trouble.
I have a ten gallon tank with a green spotted puffer and 2 dwarf puffers, and they get along great. The only problem that i have noticed is that i never really see the dwarf puffers eat unless i throw some snails in the tank. Im gonna start on some frozen brine shrimp and blood worms and see how it goes, but as far as getting along, the two dwarfs hang around each other and the GSP just swims happily around them. I havent had any problems,, but good luck!
Whetto, I thought Green Spotted puffers were brackish (despite being sold as Freshwater in places like Wal Mart) and dwarf puffers are Freshwater... therefore they are not compatable for a different reason than whether or not they fight. The GSP's can be kept in Freshwater while young, but as they grow they must have salt in their water, which is not good for the DP's.