You can do a common musk or three-striped mud turtle in a 30. They grow to approximately 4 inches shell length.
Definitely don't try keeping both the puffer and turtle together, though. The turtle will try to eat the puffer and will die of asphyxiation or poisoning, and will take the puffer out with it.
If you do want a dwarf puffer, I'd recommend the marineland eclipse system 6, which you can get for about 50 bucks, could keep 1 or 2 in there. I have the 3 gallon with a betta in it, and the temperature in it stays up very reasonably with no heater. It has a pump and light in it and that keeps the temperature up fairly well. As long as you keep room temp up above like 70ish.