Our flock of one orpington, two sussex, and one austrolorp will be perfect for her. All of them are sweet, gentle birds.
So I have been doing some research about adding a new chicken to an existing flock. I will need to keep the new chicken isolated for a while to make sure the birds will get along. Being winter in Montana make isolation of one bird more difficult.
This is an excerpt from City Girl Chickens:
Ease Them In
One of the best ways to introduce new chickens to your flock is to do it slowly. If at all possible, place your new chickens NEAR your old chickens, but not WITH them. For example, when I introduced 2 new chicks to my flock, I placed them in an upside down crate inside the hen house. My chickens could see and interact with the new girls, but they were blocked from being aggressive because they couldn't quite get to them.
All the chickens could observe each other from a close, but guarded, distance. And it seemed, also, that the crate in the middle of the chicken coop caused more curiosity not the feeling that their turf was being invaded.
If possible, keep an arrangement like this for at least a couple of days, even a week.
In the meantime, stage some introduction 'play date' times for your flock. If you have a run, or yard, bring your old chickens out to meet the new ones, but stay nearby to break up any particularly brutal fighting (you don't want to break it all up, as there is the re-establishing of the pecking order that will naturally occur, but you also don't want a bloodied pulp or dead chicken on your hands.)
Do these meet and greet sessions a couple times a day, after the first day (where they've already had time to get to know each other with a barrier of some sort between them), and every day that you have them separated from each other.