It's always good to have more than one "newbie" - it distributes the picking! I hate just having a single bird to add, I feel like I just put a live target out there for the othersI am working on intergating 4 9 week olds with 5 19 week olds. I sectioned a covered part of the run and put a temporary shelter for the younger girls to use. They can see each other and come to the dividers to check each other out. The older girls are far more interested in the young ones than vice versa. When I let the older girls out (or the young ones) into the pen to scratch in the grass and leaves I leave the others in the big run area that is next to the pen so they can see each other. I will probably continue this arrangement until the 9 week olds are closer in size and then see what happens. Wish me luck.
You have almost as many new birds as the established flock, I think it should work very well...