First of all, WELCOME to BYC!!
You need to introduce them very slowly. If you have a way to separate her so that she can see the others, that's the perfect way to introduce them. It may take a few weeks, but the longer intro. period you can do, the easier it will be for her. Do you have a dog crate? They work well for this. When you put them together, make sure she has a place to go that none of the big girls can reach her. some folks use an old screen door leaned up against the side, I've used an old wooden pallet; some place she can get to where she feels safe. Some folks put them in the chicken coop in the middle of the night after the intro. period so that when the chickens wake up, they are all together. I've not had the courage to do this. There will be some pecking, but as I said the longer intro. period you can give her, the better. Bless you for getting her back to a healthy stage, and you're right on the age; it's typically 6-7 weeks when they are just feathered out. GOOD LUCK!