First consideration - are you mixing vaccinated and unvaccinated stock? Lost half of my flock because of this one year and now buy hatching eggs not birds. On the rare occasions I need to buy in a new cockerel, say, he's quarantined for a month and then a "sacrificial" bird, such as an old hen of no particular breeding distinction, is run with him for a further month.
I normally only have to mix this year's POLs with older birds, all bred on the farm and unvaccinated, and I put them all into a completely "new" house at dusk, so no established birds feel bound to defend "their" territory against newcomers. I put in an extra feeder and drinker for a week or so, so the dominant birds don't monopolise them and cause those lower down the new pecking order to go hungry and thirsty. Keep a close watch for feather pecking and spray with a/b spray straight away. Remove the culprit, if you can identify her, and cage her on her own for a week then reintroduce her, by which time she should have slipped down the order, but cull if she starts doing it again. Provide plenty of bales, old tyres and so forth so that birds low in the pecking order can escape from bullies.