My buff orpingtons and RIR hens have been raised together, and seem to do really well. One of the RIR hens is at the top of the pecking order, but because the buffs are so much larger, they don't get picked on too badly.
We bought 4 adult Buff laying hens. Now our RIRs and BRs are being turned out with them. There was a little bullying by the buffs at first, but now everyone gets along.