I have three hens currently in my coop. Sexy is 1.5yrs old, and the two ERs are 5months. I have 6 two week old chicks and 8 more eggs in the 'bator due to hatch new years day. Timeline wise, how when should I integrate them?
Let's see. The present babies will be around four weeks old when the newest chicks hatch. They'll be too big to be able to safely put the day-olds with. They will need to be partitioned.
I have a strong conviction that new baby chicks benefit tremendously by being raised within view of the rest of the flock, and this goes for your present two-week old babies. If it's at all possible, I strongly recommend rigging up a brooder in a safe pen, either in your coop, or in the run as long as it's secure from predators and weather.
If you move your chicks now, then in a week, you can open up chick-size openings in their safe pen and they can begin to mingle with the adults. They will run back inside their safe pen when they find the flock dynamics burdensome.
Come January 1, when your newest hatch, you can install them in a brooder in another safe pen adjacent to the other chicks. They will all grow up within sight of each other, and you will be able to let them mingle together when the newest are two to three weeks old, allowing them all to utilize the chick-size openings (around 5x7") to mitigate the adult pecking order.
This is how I manage chicks and larger chickens and it works beautifully, and merging them all later is a breeze.