6 weeks is a hard age to start mixing. They have established their pecking order by then. I would do as haemony suggested. That should be a good answer. If they were closer in age or size it would have helped too. Size when that young is so different. I usually wait until my young hens are about the size of the older hens when mixing them also and letting them be beside each other and get use to seeing each other before mixing. Also taking the pecking hen out for a bit will help. Then they have to make a new pecking order. It is a tricky part to mix flocks after they are 6 weeks old. Gloria Jean