What you should do is begin introducing the two groups during the day, first in separate sides of the fence if possible, or all together free-ranging on the lawn. At night, put the chicks in the coop in a dog cage or separate area so they all sleep together and 'bond'. Then, after a week of this or so, if all is going pretty well, you can try putting them in the coop all together, with hiding areas for the younger chickens, like straw bales, and feed stations so they aren't bullied out of eating.I have had success with this.