Eight weeks is old enough to merge them with the adults. However, if you don't want them to be seriously bullied, you need to provide them with escapes.
I merge my chicks with the adults at age six weeks. But the little ones have a "panic room" to dash into when chased. It's just a small fenced off section in a corner of the run with pop holes too small for the adults to fit through. The chicks' food and water are inside so they don't get bullied away from the feeders.
In addition, I cut small pop holes in the run fence every where there's a possibility of a chick getting cornered. I have a run with several additional wings, so a chick can find themselves trapped in a corner with no means of escape, thus a target for some serious pecking. In those spots, a chick finds a small pop hole and can quickly escape.
It takes very little time for chicks to discover these safety features and to navigate them at full speed. I've successfully merged many sets of chicks this way with no problems with bullying.
As for the coop, I've found that having simple partitions on the roosting perch so that the chicks can roost in their own spot without being pecked is all that's required. I slip old Rubbermaid tub lids over the perch, after cutting a slot in the center, and staple them to the perch support. In the morning, the chicks are always the first to de-perch and race out ahead of the adults, so they figure out quickly a routine to stay safe.
