Well not quite SE brackish, but maybe close enough.
I have been keeping South Texas biotopes for a while, both fresh (SG=1.000) and hypersaline (SG=1.026). I think brackish is from 1.003 to 1.005.
What I have found down here is that the hard tapwater has been excellent for some of the brackish and salt water fish I have caught. The water is KH=6.5, GH>20. I have native sailfin mollies, gulf killiefish, sheepshead minnows and silversides that do very well in our Corpus Christi tap water. The plants that I have found such as Ammannia latifolia, Polygonum and Bacopa also do well in fresh water. I have never had to add salt to my tanks for the health of the fish or plants.
See:
http://users.ev1.net/~spituch/a.html
I think the salt would reduce the number of plants you could use. I am going to order a good book that lists all the plants of the Western gulf coast called "Aquatic and Wetland Plants of the Western Gulf Coast", by Charles D. Stitzenbaker, available from the Texas Parks and Wildlife. I saw it in the library, and it gives the maximum and minimum salt concentrations for most of the plants. I was able to ID a couple of very strange salt water aquatic plants by just leafing through the book.
Check out:
http://www.tpwd.state.tx.us/news/press/index.htm
Tell us what you are thinking of doing, and what plants and fish you may use.
Steve Pituch