I have recently introduced 5 (4 x 16-18 weeks and a 3 yr old- who was virtually free as told wouldn't lay, has layed a huge everyday bar one) to my existing 4.
Several things...
Despite keeping my new ones seperate for 48 hours thay have spread disease into my flock, cue several vets bills and will not be able to eat the eggs of existing layers for a while.
My 4 were pecking chasing V seriously, head hen was getting on their backs and flying into the air to ensure removal of feathers. The 3 yr old stuck up for herself but the young ones didn't.
I made a small 'pen' within my main pen (which is 9x15)and any chicken caught attacking was put in there. It had a perch, food, water and nestbox. So the existing chooks were in there and the new chooks were exploring and finding their feet.
At night I put them all into the house, when let out as soon as pecking started offenders were put into sin bin.
After one week things were much better, 2 weeks the odd bit of chasing etc.
BUT boss hen still will give the odd peck, she will not go to bed until all in, she will chase esp if food about and I am going to have to make an area in my outdoor run which has a roof as the boss has been keeping the babies out in the rain.
It was tiresome and a bit stressful, but they have all survived and pecking was never so bad that I saw skin.
Good luck.