If you have room to add a small wire cage, I would start out with the new one in a cage for a week or more so they get used to seeing each other a lot, or you can partition off a small section of the coop with a bit of wire. Then open it up and let them mix. Watch to see how they react when you first let them mix, and make sure there is plenty of room for the new addition to get away from the others when you mix them together, just in case. It depends on how the chicks personalities clash. I had a bantam cochin who was two weeks older then the rest of the silkies and cochins and I couldn't mix them until they were six or eight weeks because he was such a bully. When I was hatching this spring though, I could easily integrate younger LF chicks with older with no problems even though there wasn't huge amounts of space because they didn't pick on each other.
