i am sure there was no intent to make you feel stupid! the mod may not have known quite what direction you were headed in. good thing is, in short order you got the info you were after.
cory's can handle some salt. as they have a skin vs scales, its best not to salt super heavy, but when it comes to treating illness, killing off the illness takes priority over minor skin irritation.
in my opinion, way too much. i should add to this. if you really want to change out that much water, it would be much better to do 30% a day for 3 days. if that still doesnt get things where you want, give it a few days then do another 30%.
i think i see the problem with this set up, its got an odd number. you cant successfully breed fish with an odd number, better get 3 more so that you have a nice even 20.
adding outlets in the garage with a fish room in mind, id make the first one a ground fault outlet. are you adding on to an existing string of outlets, or running from the breaker box?