Here is the idea that worked for me. Follow this at your own risk. Make sure chick is close in size to those in the flock or full grown before trying this.
Separate the single chick from the main flock. The flock must not be allowed to come in contact with the chick and visa vera. They must be able to see, hear, and smell each other.
Allow single chick to be in his home for about 72 hours then place chick in with others.
Monitor coop intently for 30 minutes. Watch closely for ANY aggression from ANY ONE.
AT FIRST SIGN OF AGGRESSION place chick back in its home for 1 more day. Repeat until flock is integrated.
It is really not that hard.
When the chick is old enough to handle outside weather, place a coop within a coop. Find a dog crate that is big enough to allow the chick to walk around, house feed and water, and room enough to stand up. Place the chick in crate and place crate into the coop. Or use chicken wire to create a room for one in the coop.
CAUTION!! ADULT HENS ARE THE ONES TO WATCH. THEY CAN HARM THE NEW CHICK QUICKLY IF THEY HAVE NOT WARMED UP TO IT.
YOU MUST WATCH FLOCK TO BE SURE NO ONE IS GETTING PECKED LITERALLY TO DEATH.
The adjustment period is not that long. Some scolding from a hen is natural to set up the pecking order. But true agression must be prevented. Just try again. Separate for one more day.
I used this proceedure to introduce a lone hen to a flock of bantam and standard 6 week old chicks. I did this is the house. I dedicated one bedroom to the chicks. We made 4' square x 8' long outer pen. We placed a smaller coop from my chick-n-hutch inside the big coop.
I introduced 2 roosters to the flock as well using this proceedure. Now I have to determine when to put them all outside. I hope this works fo,r you like it did me.