They are 100% freshwater, yes. If you read any of the articles about this scat, it will tell you that too. I had one and I kept him in full freshwater for years without any problems.
Rarefish had one that got up to 11-12" I believe. He said in another thread that he moved his to a brackish setup when the fish got older and that caused another growth spurt. The ones I've seen pretty much maxed at 9" in freshwater so you could argue that they'll be fine in full freshwater but moving to a more brackish tank is more beneficial to them in the long run.