I've read (on these threads somewhere) that salinity tends to fluctuate more with a smaller tank due to evaporation, so a larger is always better in that aspect. I guess it depends on what your future tank inhabitants are going to be. Also, in a five gallon the bioload is going to be a big issue for you, also depending on the fish.
there is a wealth of BW plant info on Google just search Brackish Aquarium plants. The only two that are actually doing well in mine are the water sprite (a floating plant) and the anubias. Had to remove the ancharis as it never seemed to really take off (lots of sad looking leaves).