my GSP was in SW at about 1.5" he always seemed happier with increased salinity so i pushed him to full salt. he had been living happily in SW for about 6 months, but he jumped out of the tank a few days ago... i found him all dried up. it sucks finding your puffer on the floor. cover up all the holes. mine got through a space only as wide as he was. gotta give it to the little feller, he had some aim...
you'll need a min of 30g of tank, good filtration, and lots of Live Rock will help. 1.5-2lbs per gallon. and a few powerheads for water movement. my puffer used to play in the flow.
i had my puffer with corals no problem. yours may be different though... i had mushrooms, button polyps ect on my rock and they where never touched. even the anemones werent 'investigated'. but id wait if any corals spring up on your LR before you buy any. there was even a prawn in there that was supposed to be food... a nem got it before he realised that the prawn is what i fed him.
try to keep them full (as much as you can, its a puffer after all), and happy, then you should have no problems. re-arrange the decor now and then to stop it gettin bored and you may be able to have tankmates. all of this depends on the fish though. mine ate a dianas hogfish and a pacific dwarf angel. he would only co-habit with a common clown, and never had any problems with either one. they spent most of their time together infact...