I've never used medicated feed or ACV, but I do Fermented Feed. This summer when I had a broody raising 3 babies I worried obsessively about them developing cocci because I had successfully treated it in my flock about 2 months prior. So I watched them carefully for bloody poop or signs of stress, and for a week or two I added a small dose of Corid to everyone's drinking water just to be safe. They are grown up now and doing fine.
I know many people use the medicated feed successfully but personally I don't, because it works by reducing available Vitamin B (thiamine), which baby chickens need for development. I can get the same effect with a prophylactic dose of Corid in the drinking water.