This is an excerpt from my BYC page concerning adding to your flock. Hope it helps:
Introducing chicks to adults: Do not introduce chicks to adult chickens until the chicks are fully feathered and as close to the same size as the established flock as possible. BYC Member "Pumpkinpup" discovered "that as long as the chicks are still making baby noises, dont put them with grown birds!" She had three bad episodes in her early chicken experience, and that was "enough carnage to make a believer out of" her. It is absolutely safest to wait until the chicks are sixteen weeks old.
If you MUST introduce younger chicks to adults, here is a method which succeeded for BYC Member "Ruth". She put some chicks in a pen (a Chick-n-Hutch) inside the run with the "big girls". The big girls all came to say hello and check them out. For the first two weeks, she let the babies out into the closed run while the big girls were out free-ranging. Then, under Ruths supervision, the chicks stayed loose in the run when the big girls returned to the run for feedings. She says "the big girls never bothered the babies and the babies were really quick to learn to run and stay out of their way". After several weeks, in the evenings, Ruth left the chicks hutch open so the chicks could escape into it when necessary, and to chose when to put themselves to bed, and Ruth would come out later to close up the hutch. One night Ruth came out to close the chick hutch and they werent in there. She looked in the big coop and the chicks were snuggled up on the floor with her dominant hen. From then on, the chicks were treated just like the rest of the gang and allowed to free-range the property and come and go as they pleased.
Introducing very young chicks to other young chicks (during the brooder stage): Heres a method used by BYC Member "Davaroo" with some degree of success (paraphrased):
"Keep them apart until nightfall. When the group of chicks in the brooder are all settled down, slip the new chicks in as quietly as possible. In the morning, turn on all the lights and make a big commotion. Fill the feeders and waterers with a big, messy fuss. Your little peepers will be so worried about the commotion youre making and getting to the freshly placed food, that they will forget to fight very much (at least not more than usual). Being flock birds, chickens flee danger together, and they feed together for the same reasons. These activities are "bonding" for them."