Seriously my friend, putting *one* scat and *one* mono is a 50 (US?) gallon tank isn't a great idea. Scats and monos are schooling fish, and while monos are sometimes a bit spiteful to each other, they aren't nearly so nervous in groups as when kept on their own. With that said, a group of 4-6 would need something around 200 US gallons. Scats can do fine on their own, but they're huge beasts, and in a 50 US gallon (190 litre) tank it would hardly have space to turn around.
Why not go with SG 1.010, fill the thing with tufa rock rubble for the moray, and leave some swimming space at the front for something else. A juvenile Arothron hispidus would be fine provided it matched the moray in size. After a couple of years, trade it in. Since A. hispidus prefers (though doesn't need) marine conditions as an adult, this is perhaps the best solution for the fish, too.
There are some other peaceful brackish water puffers but getting them is more or less difficult, and depends on how good you are at identifying fish. At up to 30 cm, Arothron immaculatus is one option. It's pretty tolerant of tankmates. It's main failing is lack of colour (basically grey). Arothron manilensis is another puffer of similar size but rather better looking. Not that common in the trade, though. Again, you'd need to trade these in once they were about half grown. I'd say a 15 cm puffer would be about as big as I'd want to keep in a 50 US gallon tank.
Among the sharp-nose puffers, Canthigaster compressa is a brackish to marine example. It's a social species to some extent, normally found in pairs. Might be a bit small for life with an adult moray though.
To be honest though, I'd recommend against moray eels as "community fish", end of story. They just aren't all that reliable. Regardless of how peaceful or not their tankmates might be, morays sometimes just turn around and eat them. Spaghetti eels (Moringua raitaborua for example) are scaled down, insect-eating moray eels, Much easier to keep in groups and completely peaceful. Could easily be kept in a lightly brackish water tank with South American puffers (seen this combo, looked good) or figure-8 puffers. Would certainly work well with Canthigaster compressa if you raised the SG a bit and wanted some colour.
Cheers, Neale
Your best bet would be one moray and one Monodactylus sp and one Scat. NO Puffer. Eel like fishes seem to frighten puffers and make them go beserk. But perhaps a saltwater sharpnose puffer would work though. Then you would need a salinity above 1.015. A dogface puffer or a puffer in its genus would never fit in only 50 gallons. You have to treat brackish as a specialized saltwater tank. Be mature, stock very lightly.