Salt is generally good for shrimp in appropriate doses (I know Brine shrimp require heavy amounts to hatch).. I'm not too sure about the temperature effect on them tho..
Sounds like it's pretty bad, But keep the temp up and you will see the white spots fall off.. If the temp is at or above 87F the life cycle will end there, (no more white spots appearing, just falling off)
Yes,
QT is a good idea, esp. if the
LFS you have is selling fish that are visibly infected. I generally inspect all the fish in the tank before I buy 1, if there are other fish with white spots, or sores, then you know the fish you are getting has probably got it.. but even then,
QT for 4 weeks is generally a good idea..
Temperature is the treatment, salt is there to help the fish out, also adding an airstone or 2 to the tanks could help with lower oxygen levels (if not, and you have
hob filters, then you can drop the water line a bit to make a splash, this will aggitate the surface and allow better airation.