Whatever you plan to mix with GSPs the answer will be along the lines of "it comes down to the individule puffers". Some people have had success, some have not.... some have had a successful mix for 6 months only to find one day their GSP has killed his tank mates.
If you think your guys are placid then give it a go. However their tank mates should idealy be bigger than the GSPs.
Wow.... not a bad roundup on dozens of posts I've read on this
You cant keep two gsps in a tank that small if you want to keep two together you need to dig a pond. If you had two fullygrown GSPs you would need to divide that tank to prevent fights which is just about your only option in a tank that small.
I dont know why you insist on keeping two GSPs in a tank together Breeding perhAPS BUT YOU NEED A LOT OF WATER FOR THAT.