Polish are always a challenge in a mixed breed flock. Their crest attracts unwanted attentions. Having a single polish bird can make things much worse. Multiple polish can help so the crest isn't such a novelty.
Even under the best of circumstances pecking can happen. Mine I keep a close eye on during the molt as new feathers will spread out and skin will show which attracts other birds. When mine were chicks I had left them in their brooder just a week too long and the pecking started. I moved them out to the shed and the pecking stopped.
I unfortunately have seen what other birds will do to polish if the situation isn't right, so I always take it seriously. I'm hoping others do too. Nothing like finding your first polish rooster dead with a hole pecked into his skull because I needed to lock my chickens in their pen for a week.
