If the chick already has cocci, then (from what I understand, so I may be wrong) giving the medicated feed won't help at this point. You will have to give some sort of medication to the chick (by hand if he/she isn't eating/drinking) in order to treat the cocci.
Chances are your adult birds are fine. Chickens build a resistance to cocci, so unless you see symptoms in the adults, don't worry about it.
Chances are your adult birds are fine. Chickens build a resistance to cocci, so unless you see symptoms in the adults, don't worry about it.