To ensure everyone knows what we mean, "salt" here is common salt, sodium chloride, such as sea salt or aquarium salt (not table salt).
While livebearers can handle sodium chloride (common salt) better than many other fish, it is still detrimental (or can be) so its use in my view should be restricted to absolute necessity. From your description, that is not what I see here, so my answer would be no to adding common salt at this time.
Flashing can be caused by many things. Usually it is the first sign of a gill issue such as a parasite like ich; ich first attacks the gills, and we do not see spots, unless the fish are unable to cope and it worsens. When fish are stressed, their immune system is significantly weakened and they become more susceptible to issues like ich which in the absence of stress they would easily shake off. There is evidence, and considerable support among the hobby, for the view that ich is likely present in many of our tanks (it can easily enter with new fish even if the disease is never seen) but never becomes an issue if the fish are healthy and free of severe stress. What you have recently gone through with your water and the new tank is enough to stress out any fish, so that is likely the issue here. I would wait it out. If you see spots, and I don't mean just one or two, but indications of more, that would be a different matter and heat with (or without) salt could then be considered.
I quarantine new fish in a planted permanently running 20g tank, so the fish are going into an established biological system. Sometimes I see a few flashes initially, but these tend to become fewer and even non-existent within a couple weeks. Stress is again the culprit.