Personally I wouldn't add salt to a tank with Corydoras or plecs in- some species can tolerate it, some really can't, and I wouldn't want to take the risk.
Most plecos and cories can handle more salt than they get credit for, as long as it's not for a long period of time. Over time, it can cause stress, but I frequently use it with great results, up to a 3% solution, then immediately the next day diluting by 1/3 and continuing that for a week to remove the salt. That gives you an effective dosage to control the parasites and also minimizes the stress. The problem that most people run into when adding salt is that they've already got some in the water and then adding more can run you into the danger zone. I never add salt unless I'm specifically treating, and then I add a large dose (usually a teaspoon per gallon, added daily for the total gallonage to increase the concentration, after a 1/3 water change to remove any tomites and keep the water quality optimal). I recently acquired a microscope in order to better determine the causative factors for losses and I definitely think most people don't use enough salt to make it actually beneficial when they add it and then they leave it in the tank longer than is healthy for the fish. That's just my opinion though .