Theres a lot to adding new birds into an existing flock. You should have them set up next to the flocks pen for a while. Yoy can also rotate pens at night and add them in on new territory. You can also seperate that dominate hen when you do for a feq days. Theryre going to have to figure out the...
Being a bigger slow maturing breed they can take longer to start laying. My oldest was 8 months but they can be typical at 6 months. Someone else here might have different experiences.