Hey there - I know you ae stressed about your new girl, but take a breath
If she is eating and drinking and still bright, you have a little wiggle room. Right now, the only thing you can do is encourage her to eat and drink and keep her warm.
First - to clarify - quarantine means NO contact with your other birds, not even breathing the same air. In the future if you can, you need to COMPLETELY seperate flocks for at least 30 days. I know it's hard, but that way you don't have to worry about your new chickas passing something on - or even the other way around! Sometimes home flocks can carry yuckies that they have built a resistance to that they pass on to the new chickas and...well, then it gets ugly.
I an new to chickens, but not new to birds so I don;t have any suggestions on antibiotics that you can use but I know the list members should have some good suggestions for you. I am sure your local feed store shoud have something that oyu can use and it shouldn;t be too terribly expensive.
As far as your other chickens, keep a close eye on them. It only takes one sneeze to spread a disease and the incubation period (or the time it takes for the bacteria and nasties to actually cause a sick bird) can take anywhere from one day to thirty days to show up. That's why it is recommended that you don't put them together for at least 30 days - sometimes it takes that long for a disease to show up, even if the carrier bird (the one that brings the disease in) looks perfectly healthy.
I would keep your new girl warm, clean and surrounded by lots of good food. Get an antibiotic from the feed store (whatever fellow list members recommend!) as soon as you can, wash your hands and change your clothes before going between groups of birds and it might be a good idea to put your original flock on antibiotics, too.
Good luck with your girl!