Okay, a different viewpoint.
Amprolium, the drug most commonly used in medicated chick starter is NOT an antibiotic, it's a coccidistat. The definition of a coccidistat is: a chemical agent added to animal feed (as for poultry) that serves to retard the life cycle or reduce the population of pathogenic coccidia to the point that disease is minimized and the host develops immunity. *dictionary.com
It keeps the amount of coccidia in a chicks gut in check, giving the chicks immunity system time to build an immunity to it.
The thing is, you have to expose the chicks to the cocci in the soil in order to jump start the whole immunity process. A body cannot build an immunity to something it's never been exposed to.
Therefore, if they were my chicks I would keep them on the medicated feed for at least 8 weeks while taking them outside.
Hope this post makes sense. I'm caffiene deficient this morning.
Good luck with your chicks.