I think I read somewhere that it is actually preferable to use SW fish meat to feed FW fish as it cuts down on the transmission of diseases. Not positive though. I don't see why it would be harmfull. I sometimes feed my FW fish SW food pellets.
I've read of FW fish eating SW fish on here quite often. I'd assume not to make it the staple of it's diet, but there's also a saying that goes along with assuming.
I've fed all my cichlids fresh tuna steak and even sushi grade tune (sashimi), of course not often do to the price and deliciousness of the fish when it goes in my mouth. But they like it and they are alive.
I have used Flounder, Sea Trout, Ocean Perch as regular diet for my cichlids, Gars, Bowfins and Piranah with no ill effects noticed. I will say that the RBP's do seem to prefer FW fish species as food though!