I would treat all of them that are kept together. The medicated feed contains a very small amount of amprolium, probably not enough to do any harm. If you are worried, then switch to a non-medicated feed for the treatment period.
Don't give any extra vitamins during treatment as it can reduce the effectiveness of the medication, but once it's complete, you can supplement that and give some probiotics, to help get their guts back in balance. Amprolium is a thiamine mimic/inhibitor, so supplementing after treatment is a good idea.