If they are getting cocci it could be something else your doing. It's entirely possible to raise chicks without medicated starter and not get cocci so if they are getting it while on a medicated feed I'd look at other possible causes. I fed my first batch manna pro, used it up on the 2nd hatch until they were 6weeks old and haven't used medicated since. Haven't had a problem with cocci yet.
I use Purina Medicated Chick Starter. Seems to work fine. I make sure that they are eating it until at least a month after they have been moved to a coop/run where they have contact with the ground and other birds feces.