I just introduced two cochins to my flock and they all get along fine (except my little dominique as she was the lowest on the pecking order and had to try and defend her status!). I have RIR, two orpingtons, and an EE.
My EE is a little 'academically' challenged so she was fine and loves her new room mates, but my buff orpinton (Queen of the coop) was amazingly accepting, as was my blue orp.
I LOVE my girls and I LOVE my new cochins and after a week of 'seeing but no touching' and then a week of 'controlled/supervised' free-ranging they are all in together and laying and doing great.
So back to your question I would definitely say orpingtons for two reasons.
1. They are closest you'll get to the cochins size so they won't be an easy target.
2. Orpingtons are EXCELLENT layers and lay huge eggs.
I guess you know how to introduce new girls to a flock but don't be in a rush and do it slowly, especially with Roos...