Do you have any reading material on this? I usually refer to this article when anything concerning freshwater ich is involved and it hasn't steered me wrong yet.
Understanding and Treating Ich or White Spot
In this article it states that: "It is a commonly held belief that the Ichthyophthirius organism is always present in your aquarium and needs only the right opportunity, such as stress resulting in a weakened immune function, to attack your fish. Surprisingly, I found no scientific data to support that claim. Credible sources state that there is no long-term dormant stage this parasite can exist in."
There are quite a few other articles that back this up. The belief that ich was dormant for long periods or existed in every tank was a long held belief that has recently changed due to increasing research on the subject.
As for salt tolerance in plants it depends entirely on the type of plant but seeing how ich can be successfully and easily treated without using salt why would you risk any of your plants by adding it?