I don't personally use medicated feed anymore, and if you want to know why, you can click on the link in my signature.
The most important thing for you to consider, in my mind, is whether and how long they have been on the ground. If your chicks have not been scratching around outside, the medication can have no effect.
In my experience and research, if chicks are going to be overwhelmed by the cocci protozoa, it usually happens after about 2 weeks of living outside, daily scratching around in the soil. The medication can have a preventative effect only if the chicks are, at the same time as being medicated, being exposed to the protozoa contained in the soil. If they are not being exposed, then all you're doing with the medicated feed is keeping them from absorbing vitamins.
I hope this makes sense and helps you make your decision.
All that said, i would buy a bag of unmedicated feed along with a bottle of Corid the next time you're at the feed store, and then just medicate them if they show symptoms of Cocci.