Figure 8s have a rule of 15 gallons per puffer, and they get about 3"
GSPs should be kept in a minimum of 30 gallons/puffer, max at 6"
A GSP will need full saltwater as adults. I imagine it's easier than salt, just because with either puffer you're almost always going to find them in freshwater at the stores so you slowly raise the salinity over time.
There are a few plants that can survive up to 1.008, or so I've read, which is good for the 8s. Otherwise mangroves are a good option, just make sure you plant them in pots or you could have a disaster.
(I haven't tried mangroves yet, but I'm really looking forward to
them)
I keep both species, tank mates really depends on the specific puffer. You never know with them. You can get a puffer that gets along with everyone, or one that's intolerant. Figure 8s generally do better in groups.
Monos and Archers are both good options for the upper level of the tank. I haven't tried Archers, I'm waiting for a large paludarium to attempt them, but I've had good luck with my Sebae. Both get large so make sure your tank can support them.
Bumblebee gobies are great, but I wouldn't risk them with my puffers.
A pair of Chromides would make nice addition to a brackish aquarium, along with a school of Celebes Halfbeaks. Possibly with Figure 8s although that's always a gamble. If it's big enough, a Dragon Goby might also work with this. 55 or 75 minimum is the smallest I'd go for that, although I'd have to look at the Chromides max size to be sure.
An impressive alternative to the aquarium is a brackish paludarium. You could do Mudskippers, Knight Gobies, Fiddler Crabs, Bumblebee Gobies.. there's some awesome builds out there you can use for inspiration. Just do a search for Mudskippers paludarium. I like the DIY builds from egg crate and foam personally.
