There are several peaceful puffers that work well with other fish. Perhaps the best for a brackish water system is the dog-faced puffer
Arothron hispidus. It's usually sold as a marine but does very well at a specific gravity of around 1.010. Marine aquarists often keep it with things like damsels and tangs, so it's hardly a dangerous fish; read up on it in a marine fish book. The only negatives to this fish are (1) it's expensive, expect to pay around $25 to $50; and (2) it's very large, reaching about 25 cm in captivity but potentially twice that in the wild.
The sharp-nose puffer
Canthigaster compressa is a smaller species (~ 10 cm), totally peaceful, and very colourful. Again, it's normally sold as a marine, but it's euryhaline and naturally inhabits estuaries and lagoons rather than reefs.
The South American estuarine puffer
Colomesus psittacus is not often sold but is available in the hobby as a special order. It's sometimes sold as a freshwater fish. It's fairly big (~30 cm) and a bit delicate. Don't confuse this fish with the South American freshwater puffer
Colomesus asellus, which does fine in slightly brackish water but not the mid to high salinities you'll need to keep things like Colombian sharks. Both
Colomesus are usually peaceful though there are exceptions; similarly, fin-nipping, while rare, does occur. Nonetheless,
Colomesus psittacus is generally considered a good community fish for its size.
Sometimes combinations of fish are unpredictable. When I was at university I tried to keep some Colombian sharks with a red-tooth trigger
Odonus niger. While socially they should have been fine, this trigger being generally considered tolerant, the poor sharks would get attacked. Why? It turned out that the clicking sound the sharks made (some sort of communication within the school) was almost the same as the aggression sounds the triggerfish made. So the trigger thought the sharks wanted a fight. He attacked. The sharks got scared, started clicking more to bring the school together, and the more noise they made, the more angry the trigger got. I had to separate them.
In other words, what works in theory can sometimes not work in practise. There are lots of wild cards in biology.
As others have said, common plecs aren't an option in brackish water tanks. I believe that there are one or two Loricariidae that do inhabit slightly brackish water, but these aren't the species sold to aquarists. If you aren't keeping puffers, then snails are an option. Nerites have already been suggested, but in addition at low salinities the Colombian "ramshorn"
Marisa cornuarietis works too. Malayan livebearers will do well in anything up to half strength seawater, maybe slightly higher.
Many of the algae shrimps are also brackish water tolerant, but identifying which species is offered for sale by your dealer is tricky. The Amano shrimp,
Caradina japonica for example,
breeds in brackish water. Needless to say, puffers will eat small shrimps, as will predatory fish like Colombian sharks.
Cheers,
Neale
and they breed in brackish water)
screw the puffers; you know you want you some painted nerites 
)
