I have been trying to help my chicks develop a natural immunity by gradual exposure to the outside, starting at 10 days old (to allow their vaccinations to take effect). The medicated feed will help prevent the chicks from getting overwhelmed by cocci if they are exposed. If they never leave the brooder, they are not exposed, and you are unnecessarily medicating your chicks.
If the chicks get cocci, the medicated feed will not TREAT the infection, either.
If your chicks have been vaccinated for Marek's, they should not have exposure to anywhere that chickens or wild birds that may be carriers for Marek's have been (basically anywhere outside) for 10 days to allow their antibodies to respond to the vaccine.
So basically, if your chicks are vaccinated, and you want to feed medicated feed, you shouldn't take them outside for 10 days and you will need to feed the medicated food until after they have been outside for it to be effective.
Hope that helps!!