Thank you, everyone, so much for all your help. I have some good news and some bad news.
The good news----
So, we went to the vet earlier on today, it was at the point if we didn't try to find something, all my chicks would die. My worst fear? That it would spread to the whole flock....and that's a lot of birds.
They didn't have corid, but they had something (I forget the name) that treats Cocci, as well as a few other things. We went and got it and I'm treating the chicks with it. The older chicks are showing no sign of sickness and are all well, still, I have put the stuff in their water.
The bad news----
I'm down to just one little chick. By the time we got home, the last Sussex chick who was starting to show signs of the sickness (like half of her body kind of 'paralyzed') was almost lifeless, only a few small cheeps could be heard and she was barely breathing. Still, I mixed up the stuff and I treated her first (by a syringe) she died not too long ago.
I do have a little more good news, though.
My last chick (the last of the 1-week olds) wasn't lifeless when we got home and was only just starting to have trouble walking. She was the second one I treated, before moving onto the older chicks. I have been syringe feeding her the mixture on and off since I got home, she is in a cozy little spot with a heat lamp and seems to be getting better-- When I opened the brooder, she tried to come over to me and was cheeping. Though, I'm not getting my hopes up too high. I guess I will find out if she will make it when I wake up tomorrow.
Lesson learned. I always have mingled differently aged chicks together if I have differently aged chicks at the same time, but they are all home hatched. This was the first time I have mingled older chicks from another place with younger home hatched chicks, and as you can see it has not gone well. I am so bumbed about all my losses, they were breeds I have been wanting for ages. Oh well.