Probably too late, but....
Consider biological/medical quarantine:
BYC Medical Quarantine Article
BYC 'quarantine' search
Integration Basics:
It's all about territory and resources(space/food/water).
Existing birds will almost always attack new ones to defend their resources.
Understanding...
If you can scrape anything from under raised scales she likely does have scaly leg mites.
Post pic of legs when you get her.
What is the 'treatment'?
Adding a single bird to a flock can be very tricky.
Not sure if you want advice on that.