Hi there, welcome to the forum
You could fit one GSP in there but it may be a bit tight, two is a definate no no. It seems you have done your research as you have posted in the Brackish section so Kudos there

GSP are ofeten sold as FW but very few people relise they do very well in near to marine conditions, this means having to transfer to a set up geared for this as the puffer matures.
A pignose or T. Suvatti would be ok in that tank but as with all puffer tanks you need to overfilter to cater for the great amount of watse that these fish produce when feeding and digesting.
If you wan action in the tank, I would suggest the low brackish Figure 8, you could have 2, or FW there are more options, 6-8 dwarf puffers in a planted tank, 2 South American puffers. Other lurkers would be suitable too, my personal favourite is the T miurus.
What filter will you be running? And please ensure the tank is fully cycled before you add the puffer/s
