Even the most mannerly roos are inquisitive chaps. If you do decide to introduce the chicks to the flock by way of a secure pen inside the run so everyone gets acquainted, make certain you cover the bottom of the fencing with minimum half inch mesh screening so chicks little heads can't poke through. I had a chick get scalped by a rooster with a very inquisitive, but powerful, beak when she was poking her head through the chicken wire.
Yes, she survived, but it took her six weeks to grow a new cap on the back of her head. It was a painful lesson, to be sure.
I still introduce new chicks to the flock around age two weeks in a secure play pen. By four weeks, they are merging with the rest of the flock through 5" x 7" pop holes from their play pen, returning to safety when they get bullied and chased. By six weeks, they are seasoned enough to move into the coop with the adults, while still utilizing their playpen as a "panic room".