Did you just bring her home and toss her in? A side of the road rescue might be harbouring parasites or worse, so usually its recommended to quarantine but if she's in, she's in. Did you keep her separate from the flock so they could get acquainted before being able to touch each other? How bad are the fights? Do you have multiple feed and water stations? Hows the clutter in your run? I know it seems like a lot of questions, but there are a few things that can make this a bit easier on your flock and new girl if we know what steps you've taken and what your setup is like. It can be notoriously difficult to get a lone hen integrated but it is doable. It can take a bit of time and effort to keep anyone from getting badly injured but a rescue is a noble cause.
