Don't add SALT to a FRESHwater tank. It is not needed.
1-Cories have scales, very thick scales actually. The idea that scalation has anything to do with salt or med sensitivity is wrong because salt and meds mainly get in through the gills, not the skin. IF scalation had anything to do with it cories and plecoes would be extra resistant to it since they have thicker scales. IME cories/plecoes, and loaches/eels are not extra sensitive to salt or most meds. If you get some wild caught cories then maybe.
2-Petsmart salts all their tanks all the time and they carry all of these fish, so they are obviously not any more sensitive to it.
This doesn't mean it is okay, just that they can handle it when needed. But since we are talking about FRESHwater tanks you should not be adding any salt unless the fish are stressed. So have it on hand in case something comes up, but don't add it all the time.
77-79F would be fine. Some Amazonian fish like it a little warmer, but it depends on what all else is in the tank.