Ionic copper is more toxic to freshwater fish since there is little calcium to bind with it. However, chelated copper is not an issue - in fact, Coppersafe is used in FW or SW at the same dose.
Hyposalinity is not less stressful to the fish. Low salinity (say a specific gravity of around 1.018) is less stressful, because that is the same ionic level as the fish's blood, so they don't have to work as hard to maintain proper osmotic balance. However, true hyposalinity is lower than that, and is stressful to the fish.
Jay