Wilsoon, thank you for taking in this sweet lost lady.
I suspect you’re dealing with a number of things at once:
1. This bird is in a new home. Birds in new environments are usually very uncomfortable/insecure, and can let you know it in many ways, including screaming. There’s a temporary adjustment period. So long as you are constantly interacting with her, giving her a comfortable loving home, this phase should pass.
2) I’d bet she is hormonal. It is that time of year, and some things you describe sound very hormonal (she’s defensive/territorial with Your conure, barbering-the plucking she’s doing-is often, though not always, hormonal).
3) she’s not on a good diet with you right now, and I’ll place money on her not having a good diet in her prior home. Read through the diet thread Pip shared with you. With a super hormonal Ekkie, cut down the sugar being given to the bird, which includes fructose in fruits. Trust me, Diet can make or break a birds behavior, and likelihood of being rehomed. One of the few things squarely in your control.
Lastly, and most importantly, screaming and barbering can also be a clear sign of a health issue. On the assumption you intend to keep her, get her to an avian specific veterinarian for a full evaluation and blood panel to make sure her behavior issues aren’t health related.
And of course, make sure she has ample cage accommodations with lots of toys to keep her occupied. Foraging is a wonderful activity. And lots of training. Teach her how to target, fly to you con command. Training simple stuff like this will be very mentally stimulating AND accelerate your bonding, which can help stop the adjustment screaming a lot faster. Look up parrot clicker training - that’ll be one major key for you!