Many people feed medicated feed and then treat if the birds show symptoms (overload). Signs are puffed up sluggish birds when the rest are perfectly comfortable running around. Also you sometimes see blood in the poops, but sometimes that is too late to treat and you will loose that bird - so watch them. With a brooder situation you will sometimes not see it until the chicks have a "day outside" where the other chickens go.
I would not treat with Corid if they show no symptoms - as it blocks niacin absorption and can lead to curled toes and other structural anomalies.
I have recently changed what I do with the chicks because of the curled toes, etc.. I feed flock starter to the chicks (un-medicated) and only treat them if and when they show signs. I also expose them to the cocci early, like 3 days old (while their natural immunities are still building) by putting a chunk of turf with grass from where my chickens roam.. My mother has broody raised babies, and she has never had signs of cocci in her flock, they all build up their immunities to it when they are really young. I did this with my Lavenders and they are huge and healthy - no signs of cocci. I will be watching them closely this week and the next though - some people also say it hits badly at 8 weeks.. I have Corid and Sulmet if I need them.
My birds are almost 8 weeks old and I am still not sure who is who.. I feather sexed at 3 days old and showed 3 male and 5 females. I banded them. Now I am wondering if I have 3 females and 5 males...
but it doesn't match backwards from my early guesses either.. so only time will tell.