I can't decide for you about whether to get rid of the mandarin.
As I said, they have natural immunity, and most likely yours will not have ich. If it does get ich, you will know it within a couple of weeks, which is why I have told you to keep daily watch on the mandarin to see if you see ich. In the meantime, your other fish are in treatment, and you will know about the mandarin way before the other fish return.
The ich will not hide on the mandarin waiting for an opportune moment. If by some rare chance, this mandarin gets ich, then you will know it and be able to deal with it before your other fish are ready to return to the display. I have yet to ever see a mandarin with ich, though.
Have you ever seen ich on this mandarin? If not, then why do you think you will see it now?