One of us is confusing RUN vs COOP. My RUN is plenty big enough. It's 132' x 35'. My COOP on the other hand is not big enough to separate out a brooder area for 8 chicks where everyone can see each other but not get to them. I have enough room in my coop to put a large dog crate (42-in L x 28-in W x 30-in H) which could be used to introduce 2-3 chicks at a time. I'm not super worried about them in the run itself because there are many places for them to hide and get away.
The confusion is because you used coop/pen which I take to mean a combined coop-run, or something (as "pen" is generally an outdoor enclosure). But nevermind the semantics. Since the coop is smaller, introduce them in the run instead. My chicks never see the inside of the coop until they're already roaming with the adults in the run.