\hought I would add some Epsom Salt just to help relieve stress
Salt DOES NOT actually do this. It is a misapplication of the concept of osmotic stress.
Osmotic stress is the stress that is formed due to the presence of dissolved matter in water. Stress in the this case is used in the mechanical/engineering/physics sense, not in the fishkeeping sense. In the mechanical/engineering/physics sense, pressure is also a stress. And, in fact, in the scientific literature, osmotic stress and osmotic pressure are used almost interchangeably.
An osmotic stress does form over the skin of a fish, simply because the level of dissolved minerals in the tank water and in the fish's internal bodily fluids are different. Nature prefers the levels to be balanced, so there is a stress due to the imbalance between the levels of dissolved matter.
However, and this is the big point, the fish has lived with this stress it's entire existence. It has evolved to deal with this stress for many millions of years now. It is exactly the same how we -- humans and all air-breathing creatures -- has evolved to deal with the stress of our atmosphere. And yes, our atmosphere is applying a large amount of stress on our bodies. 14.7 pounds per square inch to be exact. Take an object that weighs 15 pounds and rest it on your leg, even better if the point you rest it on is only a square inch -- it weighs a lot! It is a large amount of force. But, we don't notice this force pressing on us from all sides because we have evolved and lived with it our entire life.
When you take this atmospheric stress away... you get sick! This is known as altitude sickness and happens to a lot of people who leave sea level and go to Denver or the Alps for a significant period of time. When you are sick, you are not looking to move into a low-pressure chamber to be relieved of the "atmospheric" stress. And, the same things for your fish -- they are not looking to be relieved of the osmotic stress. They have lived and evolved with this stress throughout their entire history. They will be worse off being relieved of this stress than non being relieved.
If the osmotic stress was sooooooo bad, why didn't every fish species everywhere eventually evolve to live in the much more salty ocean? Freshwater fish do not need to be relieved of this stress, and the manufacturers of the salt are playing on this word stress -- because we always hear about how stress lowers the fish's immune system -- to scare you into buying their product.
In particular, your discus evolved in some of the most mineral-poor waters on the planet. They don't have the mechanism to deal with extra salt in their water. You will probably do as much harm as good by exposing them to the salt water. There is good in that it will probably help kill the disease that is afflicting them, but bad in the harm to the fish themselves.
Finally, to answer you question, salt in the water may affect the medication. That is part of the reason some of the medications the saltwater keepers use is different than the freshwater keepers. It is unclear how much effect salt could have, and is probably best to check with the manufacturer to know before mixing. However, I hope that I've convinced you that salt isn't necessary at all, so mixing salt and medications is hopefully just a moot point at this time.
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