My GSP is in almost marine conditions (soon to be transitioned to full marine) and several people over on ThePufferForum and elsewhere have had great success with GSPs in salt water tanks. Yes, it's completely possible to keep these fish in full marine even from an early age / small size with no ill effects to them. In fact, I would recommend they are kept in at least very high brackish conditions from 2" or so on. Some have moved 1" fish to full salt without ill effect. In fact, many have reported that the neon green head spot generally agreed to indicate good mood / good water conditions in this fish did not appear until salinity was raised considerably. With my fish, the neon spot never goes away unless I dare move her decor around... and which point she'll turn almost black and sulk for a day or two. Stupid fish, you're supposed to like change and new things to explore! (Really, she's fine with 50%+ water changes, but if I move the plants or fake tree stumps around she goes on a hunger strike...)
If you decide to get one / move one from fresh water / brackish to full salt just drip acclimate them slowly over the course of an hour or two and they should be fine. I would make sure they're at least 1.5" or so personally, but as I said others have had success with even smaller fish than this.
I've seen you post around here a lot so I figure you already know this, but remember that despite how cute they can be you are dealing with a very aggressive species. Anything that isn't substantially faster than the puffer is likely to end up with some fin bits missing and may even be killed. Shrimp and crabs are good as dead, regardless of relative size. Smaller ones will go first but larger ones are likely to be eaten later (possibly weeks or months later) when the fish gets hungry or bored. Fish with long, flowing fins are likely to be harassed to no end as well. Unlike many predatory fish the "will be fine with larger or similarly sized fish, will only go after things small enough to fit in its mouth" rule does not generally apply with the GSP. These little death-blimps will take a chunk out of anything they feel like, even fish 5x their size.
Depending on water volume it's possible to keep more than one of these in a tank but I have never had much success; there are regular squabbles even with two or three 2" fish in a 40 breeder with lots of decor to break up line of sight. I'm currently keeping one alone in a 20 long for this very reason -- she tried to murder her previous potential tankmates. If you decide to get more than one getting them all at once and similar size helps a lot with curbing aggression, and more water volume / surface area (bottom area) is always better. They can be very territorial and all want their own hunting space. That said, there's nothing like putting a crab or crayfish in the tank and watching 5 little GSPs coordinate on the attack against something bigger than all of them put together -- and win!
Of course, personality and aggressiveness varies greatly between individual fish. My current specimen is more aggressive than most and has made honest attempts to kill everything I've ever trialed with her (likely a male given this I know, but it's a girl in my head and there's no way to tell, so...). Don't worry, they all went back to the local fish store within hours for their own good only a little worse for wear, a couple fin nips lighter. If you're looking for tank mates they generally do best with things that look nothing like themselves. When she was in very low brackish there was a pleco in there with her for a while, but he was moved as salinity increased. She never even acknowledged the pleco, never went after his fins or anything, and he wasn't much bigger than her. If there's a chance a fish may be mistaken for another GSP... that fish probably shouldn't be in your tank. Generally I'd recommend a species only, single resident tank when it comes to these just for simplicity's sake. Watching them hunt down shrimp you bought for feed and snails you put in the tank to wear down their teeth is fun. Watching them hunt down everything else... not so much.
EDIT: Last edit, I promise!
Just saw your pictures thread. If you're considering a GSP for the 55 gallon they will more than likely happily murder any and all snails in the tank and the hermit crab, then sample corals that strike their fancy on a given day. Wouldn't recommend it, though it may be worth bringing one home to trial and hoping you get a calm one. If you're starting a new tank or it's for a different tank already set up don't let my somewhat negative presentation above dissuade you. I do love my fish.