Im sure others will chime in as well but since I noticed there was no response I thought id post a comment.
I personally dont mix sizes. Bantams stay with bantams and large fowl keep to their own. Even had problems with skinny bird breeds getting picked on by the chunkier ones so I tend to keep them seperated unless necessary.
If I have to add them in I do what most people do and sneak them in at night. I put them on an empty roosting space and then first thing in the morning I observe behaviors and continue to do so for awhile each day til im convinced no one is going to bully anyone else.