I had my 4 in a 60g freshwater with my breeding pair of severums, and a small pbass, but when i came back from vacation they had all disapeared lol. they were fine in freshwater until the vanished.
The one I had went from 6" to around 7" in a FW though I do use 1 tbs salt per 5G. According to most sites they are more often caught in freshwater lakes and resivours than in Brackish areas, here is a fairly useful link, the fish seems pretty hardy and I had no problem while I kept one. http://fish.mongabay.com/species/Nandus_nandus.html
I have 4 of them and all in fresh, though I am not as fond with them as I once was. They basically are very very still untill feeding time then they are lighting fast and you barely see anything at all.
In brackish? I would do it! I really want a nandus nandus now too Ewurm. I think they are actually related to dats. Go for it, I am going to try to find one for my tank too.