30 gallon tanks are funny beasts. They're big in some ways, small in others. If you distribute the fish into surface, middle, and bottom dwellers you can get a lot of stuff into them because you usually have lots of depth to work with and also enough space for serious rockwork and planting. On the flip side, you are only working with the equivalent of 10 buckets of water, if that, and so managing water quality remains tricky, as with smaller aquaria.
It's a shame the for many people brackish water is synonymous with big, active fish like scats and monos. There are a tonne of small to medium sized fish that would work in a 30 gallon tank: gobies of various kinds, glassfish, halfbeaks, soles, dwarf cichlids, killifish, goodeid livebearers to name a few. You may need to shop around but most of these fish are regularly traded even if they aren't in your local store right now.
As JJ1234567 mentioned, GSPs and figure-8 puffers have different requirements: one prefers a low salinity (SG 1.005) while the other needs at least a medium salinity (SG 1.010) and arguably near-marine conditions when mature. While it isn't a brackish water puffer, the South American puffer will tolerate a low salinity (SG below 1.005) and tends to get along better in community tanks. Some people have SAP horror stories, but all I can say is my experience of them is that their nipping and aggression is entirely manageable and no worse than, say, a dwarf cichlid like a ram. SAPs also works well in 20 to 30 gallon tanks.
On the other hand, your tank is too small for scats and most monos. Possibly, if you could find them, Monodactylus kottelati would work since they barely reach 10 cm in length, but they are not deliberately traded and you need to be able to recognise them in batches of regular monos. Scats grow astonishingly quickly, and will go from inch-long babes to six-inch eating (and pooping) machines well within a year.
Archers should be schooling fish and usually are in big tanks and when kept in big groups (6 or more). Pairs and trios invariably seem to squabble, and I have heard of the dominant (larger) fish causing harm by not allowing the other one or two specimens the chance to feed properly. Continually stressed and hungry fish quickly succumb to diseases.
The Brackish FAQ on my web page lists lots of different kinds of fishes suitable for your aquarium. My own aquarium is about the same size (180 litres) and there are some pictures giving you at least some ideas of what you can keep. While the water is soft and acid, the theme is brackish, hence things like puffers, glassfish, gobies, and halfbeaks.
Cheers,
Neale
