That's like the layout of my roosts and I have the same problem come bed time. The old girls will want to be higher than the newcomers and bully them off the perches.. Not sure how much space you have to work with, but adding a lower or practically ground perch might help. Mine's a raised coop, so I don't have any vertical space to work with in mine. I had to sit out there and play rooster at bed time till everyone settled down; it was like putting a toddler to bed 
