I wasn't putting my chicks in with older chicks, but we had a time getting them used to the new coop. When I moved them out of the brooder and into the coop, I left them locked in for 4 days with the brooder light in there with them. I opened the door on the 4th day and they wouldn't go out, they would stand there piled up at the door and one would get pushed out, and they would scramble like mad to get back into the coop.
After a couple of days with the door open, they finally ventured outside. The brooder light was still in the coop and they would go in every night. It finally turned downright hot outside and the chickens had fully feathered so I took the brooder light out. The first night they wouldn't go into the dark coop with no light, they were all huddled up out in the run. I moved them all inside and put just a regular light inside the coop. After that I'd go out there before dark, turn on the light, all the chickens would go in at dark, and I'd go back and close the coop up and turn off the light. I did this for about a week until one night I was late getting in and didn't turn the light on before dark, the chickens were all in the coop. Now they go in the coop without light.
All that being said, it would probably be a good idea to put a light in the coop at dark for the first few days. This should insure the new chicks go inside. It seems like nighttime really freaks new chicks out that have spent all their life under a brooder light.