Hi there, GSP's should be solitary fish, although they get on fine now as they approach adulthood they will become territorial and fight. Even at this stage GSP's need a brackish environment which eliminates many fish you can keep with them, thats if they don't eat them before! They're not good kept with other fish and you may come down to find carnage in your tank one morning. The older they get the higher salinity they will require, and this needs to be made up with marine salt and not aquarium salt, aquarium salt won't give them what they need and maybe a contributing factor in their growth being slow. Always mix the marine salt with the water in a bucket and wait for it to be fully dissolved before adding it to the tank and never add the salt directly to the tank as this will cause burns inside and out to the fish.
As for Figure 8's they require a lower end brackish water, SG of between 1.002 - 1.008, you can keep them together (I keep 4) but make sure they are in a big tank and there is plenty of plants, caves, places to hide etc and that there is enough to break the eyeline so they don't fight. Some keep F8's with other fish but it's not recommended as they too are territorial and liable to eat tankmates!
Although they're both pufferfish they are very different fish with very different requirements and should not be kept together, really it's like putting a goldfish with a clownfish, the 2 won't mix and neither will the GSP's and the F8's.
Also as an after thought, be very careful to monitor your ammonia, nitrite and nitrate levels, these are very sensitive fish who do not have scales and are very susceptable to poor water quality and get stressed easily.
Hope this helps you, best wishes, Jane