Adding 6 week olds to 6 month olds has never been a problem for me. I do give the little ones a wooden box with wire on the front that they can get into but the bigger girls can't fit in the door way. They sleep in the box and I give them a feeder so they can eat in peace. With 17 little ones, no one chick will be singled out and picked on. The only area of concern would be the health of the older pullets assuming they are coming from the outside and not ones you have raised. You don't want them to introduce something to the little girls.