I raise my chicks either in the brooder in the coop or let a broody hen raise them, either hatched myself or mailed from a hatchery. They are exposed from Day 1 to whatever the flock has. I feed dirt from the run to my brooder chicks to expose them by the second or third day in the brooder...
You might try following the link in @Stravager 's post above, post #4. It should answer most of your questions about medicated feed and Coccidiosis. I'll try to give a few highlights but it can get confusing with all the "if's.
You need to check what medicine is in medicated feed. Usually in...