I am starting this thread due to the apparant lack of brackish keepers here.
Most fish living in a brackish enviroment are quite hardy to changing water enviroments. This is due to their natural environment has changing tides which brings an ever changing salinity level. This changing environment also has the added benefit of killing off pathogens. Some fish live in estuaries and some live in the salt marshes. The difference here is that a salt marsh as any other type of marsh has very little water flow so these fish live in slow moving water with a closer to stable salinity level and lots of plants and driftwood. An estuary has a variable water flow from one side of the river to the other and variable salinity per foot. There are plants living in an estuary but much less than in a salt marsh.
brackish fish include Mollies, guppies, some breeds of goby and puffers, scats, and chromides, as well as many others. The trick here is that alot of breeds start as brackish for the safety of the younger small fry to grow in an area with less predators and as they grow migrate to a fully marine environment such as the green spotted puffer. Research is the key.
The natural substrate will be mostly sand with gravel mixed in and rocks and driftwood to hide in.
Plants is one area I am still researching. I have found from personal experiance that some very hardy plants such as anacharis, amazon sword, and java fern can tolerate a lower end salinity if acclimated slowly. This means increasing the salinity by one extra scoop of salt added after water changes every month per month. At first it will kill off particularily week plants and temporarily hault the growth of others. If aiming for higher end salinity some species of macro algae can tolerate lower than marine salinty which would give more color options
Lower end means salinty of 1.002-1.008, higher end means 1.010-fully marine.
Make sure that the additives and equipment you use is for both marine and fresh water use. Marine salt is better for this use due to the added PH buffering. A hydrometer or refractometer is reguired to keep track of the salinity though some species can tolerate a small amount of salinity change