Generally speaking puffers do best as a species only tank or as a singleton. Of course that will vary with each fishes temperment. Some will tolerate other fish some won't. Even if some will tolerate other tankmates it may eventually turn on them.
What size tank do you have? The minimum recommended tank size for 1 green spotted puffer is 30 gallons. They will also need brackish water, transitioning to high-end BW/salt water, to survive. So you will need to to buy marine salt and a hydrometer/refractometer to measure salinity.
80 gallon FW
:3 clown loaches, 2 angels, bloodfin tetras and 1 albino BN pleco,1 Farlowella.
46 gallon FW
: kuhli loaches, otos, black neon and black phantom tetras, 3 pitbull plecos.
50 gallon SW
: 1 green spotted puffer