another suspected cocci - slightly different questions

I have a cochin, about four months old that is having bloody stools. she is slightly more listless than normal, is picking at her food, and drinking water.
She was an only chick as she was too small to go in with the big girls alone, and she came with what turned out to be a roo. Had to give him back, so bought another young chick on 7/1 and have been keeping them together, and away from the big girls.
I just read a couple other posts with similar symptoms and I'm wondering if I should treat this as cocci, and where would I get corid? How quickly does this tend to be fatal? I'm thinking it's too late for the feed stores, so tractor supply is my only option for tonite, or do I have to get it from a vet?? And if I get it, should I give it to the other chick too? What about the big girls? As I said these 2 are separate, but they go out in a crate in the yard - obviously the other girls can't get in the crate, but they cruise around it...
Thanks for any and all help/advice! I love this site and have been checking it out since before I got my first group of girls last spring. I wish there was something like this for horses too, as I am now a new horse owner too!