Thank you both for the sound advice! Here's an update:
So night before last, I put the young pullets in the coop after the hens had gone to roost for the night (about 9:15 pm), but they were still awake and alert. It was light outside and we have a light in the coop so we didn't "sneak" them in. I watched them for about 20 minutes paying particular attention to one of our RIR's who likes to "patrol" the roost area. To my surprise, no issues! I went back out after 10 pm when the light had gone out and it was dark and found everyone was quiet. The pullets were on the floor and the hens on the roosts..so I let them be. Next morning, my husband said the pullets were nestled on a "stump" below the roosting board and everyone seemed well-behaved and the hens seemed unconcerned to have the babies in their space.
Last night, I did the same thing. When I went out to check after 10, 3 of the young girls had made their way up on the roost platform with the big girls. Two were even nestled on the smaller 1 ft roost with one of our sweet Auracana's! I had two in my nesting boxes below so I carefully moved them up on to the large center roost and watched quietly...again, no issues or aggression with the older hens. Got up this morning and everyone was busy eating with no issues! YIPPEE!
So my next question is...do I need to worry about which food the young pullets are eating? I thought I read that they should not be eating layer pellets before they start laying?