1. How old should the chickens be when I introduce them into the main flock?

We recommend that you wait until the new chickens are nearly the same size as the mature chickens in your existing flock.  Install a temporary partition in your existing coop so that the new chickens can live next to your main flock. For the partition, use something that the chickens can see through and preferably something they can also hear through and smell through, such as mesh or chicken wire.  After a few weeks, remove the partition and let the new chickens mingle with the existing flock.

2. Young chickens require a different diet than mature layers. How do I accommodate this?

If you wait until the younger chickens are nearly the same size as the mature chickens before you combine them into the same flock, they will be ready to eat the same feed as the mature chickens, so having separate feeds will cease to be a concern.

3. The hens in my existing flock are molting?  Will it stress them too much if I introduce chickens now?

If you follow the instructions above, then introducing the new chickens will not cause much additional stress, so it is not necessary to wait until the molt is complete.