IAR: This sounds like a really cool way to do this. What do you use for "Mama Heating Pad?"I use a folding wire dog kennel. I had to line the sides with 1/4" hardware cloth so the chicks can't just walk through the uprights and then fling the pine shavings everywhere. But I only had to do that once.
What I like about it is I can put it on the big counter between my kitchen and family room. We're together all the time when I'm working in the kitchen or watching TV so they get used to me. I'm not looming over them and being a terrifying presence. I can reach in easily to clean the feed and water. They come right to the door if my grandson or I approach. There's lots of room for Mama Heating Pad, a waterer and a feeder (though probably not 20 chicks!). I put in little branch roosts with nylon zip ties.
Then when it's time for them to go out to the run, I take them kennel and all. They have their own protected food and water while the big girls get used to being around them. I put the kennel next to the big girls' feeder so they get used to being close but neither group of birds or their food source is threatened by the other.
After 3-4 weeks out there in the shared space the chicks are pretty adolescent and hardy so I open the door to the kennel and let them decide when they're going to venture out. Initially, they'll spend their days in the general population but go back to the kennel to roost when the big girls go up to the coop. But when I feel like it's time, I take the kennel out and fold it up for storage. The adolescents will still huddle together in the run at night (it's perfectly safe) until one day they just decide to follow the older hens up to the coop. They'll huddle in the bedding on the side opposite the roosts for a while but in time they fully integrate. They find their spot on the roost and they're a member of the flock.
Since I've started doing it this way, I've had NO mean girl nonsense. It's been great.