I don't think I'd add a puffer to that tank. You're about at your limit for a 10 gallon tank fishload-wise, for one thing. And although dwarfs are a freshwater puffer (unlike the figure 8 you mentioned, which really needs brackish conditions to thrive), they would chew up the fins of your other fish.
You may want to consider getting another 10 gallon tank for some dwarf puffers alone. Keep in mind, though, puffers can be demanding fish. If you're new to fishkeeping, you might want to wait until you've got some experience.