The logic is correct but the practice is not necessary. While a low SG allows water to carry more oxygen, the oxygen should be generated from vigorous surface agitation, i.e., pointing power heads at water surface, airstone, etc.
Cupramine cannot be used with hyposalinity, and the main reason is that copper is more toxic in low pH water, which happens often during hyposalinity. That is why Seachem states on the cupramine bottle that if cupramine is used in freshwater, the dosage needs to be half (0.25ppm) instead of the 0.5ppm in marine water.
That being said, water at 1.015-1.017 generally has enough alkalinity to keep pH in the 8.0-8.2 range (this from my personal experience), so theoretically cupramine can be used in there. However, like MrTusk stated, 1.015-1.017 has no merits in terms of disease treatments, so there's no point using that salinity. You will be better off acclimating your new fish into seawater salinity (1.025) and apply cupramine.
