If you only have room for one more I think giving her any chicks is a bad idea. While the hen is broody she will probably be able to take care of her chick and keep it safe with your flock, but at some point a broody will wean her chicks. She leaves her chicks to be all on their own when she rejoins the flock. I've had a broody hen wean her chicks as young as three weeks, I've had some wait until over two months to wean them. From the time she weans them until they are old enough to make their way into the pecking order (Usually about when my pullets start to lay) the chicks have to manage their own way with the flock.
The way mine do that is to avoid the adults day and night. During the day they stay where the adults are not. At night they do not sleep on the main roosts with the adults. If you only have room for one more you don't have room for them to avoid the adults. It takes more room to integrate than for a flock of fully integrated adults.
Another issue is that chickens are social animals. They want to be with other chickens. If you try to add only one it will not have a buddy after the hen weans it. So it will be very lonely or it could get injured trying to be with the adults.
My concern is not while the broody is taking care of it, it's after she weans it and it is on its own in what I take to be pretty crowded circumstances.
