Yes, you can convert your GSP tank to SW, no problem. I have sucessfully kept mine with damselfish, tomato clownfish (maroons are too aggressive & percs are too weak) & other fast-moving fish, like wrasses. It all depends on the aggressiveness of the puffer though.
I don't know where folks keep getting the idea that loaches (especially clowns) are OK in BW. THEY ARE NOT BW FISH! They actually come from the opposite kind of water--soft water.
LOL, keeping a BW fish for 2 months in FW & saying it's fine, when in BW they can live 18+ years is a joke! After a decade-long study by a research scientist that has been keeping puffers for over 50 years, F8 are best kept in a SG of 1.005.