The stress coat stuff isn't going to hurt if you use it. If the fish are healthy, it is not required. I personally never add anything to my tanks but prime (aside from fertilizers for my plants). The key is keeping the water clean and healthy... and the only way to do that is PWCs. I change about 50% once a week (obviously while you are cycling with fish, it takes much more than that). I think a lot of problems are experienced in the hobby when people overstock their tanks, don't do regular PWCs in sufficient quantity, and then turn to chemical solutions to fix the problems (although it doesn't fix the underlying causes/issues... it just puts a bandaid on whatever the real issues are).
The aquarium salt is not necessary either. It won't hurt anything if used properly, but I doubt it would help much either. Very few fish need it to be at their best.
If you return the fish and do a fishless cycle, there would be no need to ditch what you already have started... it should be fine.