Ive administered the treatment to more than 50 different species, including cold water and sensitive fish. Never had a problem. I agree if one has it then they all must be treated.
I haven't found changing the water to have any impact on the treatment, or the fish during the treatment. I have seen a number of cases where water changes are performed and the treatment is not effective. That's just an observation - by no means enough to claim any kind of a relationship. In my opinion it's just an unnecessary variable to introduce because the benefit does not outweigh the risk. Others will disagree with this for sure. Many a successful heat treatment has been administered in conjunction with water changes - don't get me wrong