I set my mom up with a 75 gallon tank with java ferns, anubius, and some large crypts from my tank and all have been doing well with the fancy goldfish. I think the biggest problem with goldfish is their tendency to suck on plants and uproot them (if not eat them entirely). I have heard that faster goldfish, comets or shubunkin, are the ones that really like to drag plants around. I have even been successful at getting java moss on some of the decorations with the goldfish messing with them constantly. I have an idea that they would also leave bolbitus sp. alone.
As for brackish water, I have no idea. I'd be wary of putting goldfish in brackish as well, but then again, I have not tried it myself.
Good luck with your goldfish endeavors and if there is one bit of advice it would be to tie everything down to decorations or place larger rivers stones/rocks around the bases of your plants to give them a chance to take hold.
Somebody set us up the tank!
ALL YOUR FISH ARE BELONG TO US...