I have seen people mix Figure-8 Puffers with Green Spotted Puffers.. however, after a period of time the green spotted puffers will grow larger and more aggressive then the F8's, and probably kill them. They grow to 2x the size of the F8, maxing out at about 6".. I would only allow 1-2 GSP per 55g tank, anyways.. They are messy eaters, and with the brackish water requirements (preferrably, but can withstand most salinities), it can make water changes and water quality a pain in the ass..
You could also mix the dwarf varieties.. There is C. Travancoricus, aka Dwarf Puffer.. If you can get some nice fat ones, you could mix quite a few.. I have also seen people mix these with the Dwarf Red-Eye puffers.. There is C. Irrubesco, and C. Lorteti.. I can't remember which ones, but one species is peaceful and can live in groups of 1 male to multiple females (and I have seen them housed with dwarf puffers), while the other species is extremely aggressive and will kill just about everything.
I have also seen people mix A. Modestus (Bronze Puffers) with a pack of Dwarf Puffers, but this was an unlikely match and the keeper I would say is more than 'lucky' as the bronze puffers usually have a 'trigger-like' disposition.
Most people will tell you that majority of the puffer species are incompatable.. I have seen some people keep puffers from the 'Target puffer' family togethor, but often these species aren't available, and you will have a hard time finding a mixture of species to combine.. I have also seen people keep other puffers with T. Cutcutia (red-eye puffer, sea frog puffer).. However, regardless of which combination of 'target puffers' you combine, they all pretty much lay there and don't do anything until feeding time.
My suggestion to you would be getting a pair of Figure-8 Puffers, and keeping them in a light brackish water.. They won't outgrow that tank long-term, and if you choose to keep up on maintenance you could likely do 3 or 4.. F8s are a little more docile then the GSP counterparts, and you will have alot more success keeping other tankmates, including predatory ones, with them.. I would go with 2, 3 or 4 Figure-8s, along with a Butis Butis and a pair or a group of 'Knight Gobies'.. I have also seen people mix Orange Chromide cichlids in with this bunch, but it may be a trial and error issue depending on individual personalities.. Also, an Archer fish would be a nice compliment for another predatory fish to add to the mix..However they get very large over the long-term, keep in mind..
Hope thats a start, Good luck!
Miles