Th green spotted puffers need at least brackish, higher end brackish as they age and full marine is best as adults. They will stress easier when in freshwater, immune system is compromised, and will live shorter lives. They would be fine in full marine at a young age, I think this is easiest as the tank could be set up as saltwater to begin with and the filtration in saltwater (live rock, skimmer, etc) is more efficient than with freshwater.
The 20gallon would be too small unfortunately. The 90g would be more than enough for two full grown green spotted puffers and would allow space for potential tankmates, depending on the puffers.
For one green spotted puffer 30gallons is minimum because of their size, bioload, eating habits, etc. A 30gallon, however, would be on the smaller side once at full marine levels with the space that would be taken up with live rock. Their swimming space would be compromised and lesser water volume would likely cause issues with parameters once at full size. These guys reach up to 6 inches and are big around as well, think nerf football. For one in full marine conditions I'd say a 40g would be best, better footprint, more water volume, etc. for just one green spotted.