I use a coop brooder with the Mama heat pad. At four weeks, I use a panic door panel for the brooder, to let the chicks interact with the big girls. I have stuff inside the coop and run for them to run behind (open on both ends) or go under to get away from any pecking. They went back to the brooder for about a week to sleep. By 6 weeks they were brave enough to get up on the roost board with the big girls. They usually split the big girls, by roosting right in the middle. Using a coop brooder IMO, makes integrating easier, because they are exposed to the rest of the flock the whole time they are growing. The chicks will get some pecking from the big girls(that's expected), as long as there is no blood drawn, it's normal. I have found that integrating at a younger age went better than at an older age. They are not involved in the pecking order yet, I think that helps a lot.
New set up for 2019
I changed my set up this year (2019). I made my brooder 2 ft longer(8 ft) by attaching the dog crate differently to the x pen. Now I need to make a different top for the x pen portion. I had to put a piece of wood on the crate end to cover the opening. By setting the brooder up differently, I now have 4 panic doors on my brooder.
I also started integrating this years chicks at 2 1/2 weeks old, instead of 4 weeks. I wanted to start at 3 weeks, but I start integration on the weekends and they were gonna be 2 1/2 or 3 1/2 weeks, so I gave it a try even earlier. It went well, the chicks didn't leave the coop and the big girls would come in to check them out, but didn't really pay to much attention to them, except for looking at them with a cocked head sometimes. The pallet on the floor is for the chicks to duck under if the big girls bother them. The coop feeder and waterer also work as a chick get away area.
At 3 1/2 weeks old I left the panic doors open all week, so the chicks could interact with the big girls. I wasn't around most of the time, but the chicks were good at getting back into the brooder, when they needed to.
4 1/2 weeks old today. I turned off the mama heat pad, it was on 1 the past couple nights, I also took the brooder down and made roosts in the corner for them. They were playing on them today and used them tonight on their own. I expect they will be up roosting with the big girls within 2 weeks.