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.
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.
I'll be keeping sailfin mollies and mummichogs, as well as anything else I might catch while I catch the mummichogs. I'll be going to a brackish area near Charleston, SC (don't know exactly where yet). I'll look for plants while I'm there, and I'll test the salinity there, too, but I'm looking to get the tank set up as much as I can before I go.
It's funny - the grass is always greener on the other side. I am jealous of the fish I have read about in Texas and the Midwest. Members of The North American Native Fish Association (www.nanfa.org) are planning a really big trip to Ohio. But I'm a grad student and can't go. But I won't be a grad student forever!
FYI, there is a NANFA e-mail listerve. Go to www.NANFA.org, and you can sign up. There don't seem to be any CA local groups, but check http://www.nanfa.org/NANFAregions/regions.htm to see if there is a group in another state that is near you. AND, you can join the listserve and just ask who is near you that you can go with. I've started keeping native fish only recently, and EVERYONE I've met from this club is super nice. They've taught me so much, and aren't exclusive in the least.