If you have a 30gal and don't mind venturing into brackish, 1 30g is big enough for 1 green spotted puffer or 2 figure eight puffers. I found out of all the puffers I own my GSP is the most tame and personable out of all of them. IMO
a SA puffer is bad for a first time puffer because of what puffergirl said, you need to trim their beaks manually and at least twice a year. GSPs can fed a wide variety of shellfish and have little to no trouble breaking them and grind their beak down easily. So if you think you can handle brackish, and evenually SW
(it's the beginning of a journey into SW
aquariums). The GSP will eventually need SW
as it becomes 3-4" in size (max size 6").
You're doing the right thing researching puffers before purchasing, they're aren't the easiest fish to keep and are extremely sensitive (rivalling discus sometimes). Make sure your tank is cycled before you purchase anything though, and if you do decide to go brackish, use insant ocean and when you buy the puffer have the water as FW
and slowly raise it (to not disturb the bio filter) to BW and then to marine over a course of a year or two.