How old are the new chickens? Are they mature enough to lay or still immature? Until they start to lay my pullets avoid the older hens as they are likely to get pecked if they get too close. My older ones leave them alone as long as they don't invade their personal space. That avoidance is during the day and also at night as they get ready to sleep. Once the pullets are laying the older accept them into the pecking order. If your new ones are already laying then they may or may not have a pecking order fight or two but should soon fit in.
Chickens are creatures of habit. If they are used to roosting in a certain place that is where they are trained to sleep. Sometimes when you integrate them they will move on their own, sometimes you need to help them. I wait until they have proven to me that they can share a space during the day without violence before I try to put them in the coop at night with the established flock. After it is too dark for them to see I put them in on the coop floor and lock the door. Then I'm down there at first light when they wake up to see how they are getting along. If things are quiet I'm soon comfortable sleeping in. If not, I open the pop door and let them out every morning until I am comfortable they won't hurt each other.
To catch them I wait until it is dark and they have put themselves to bed. It is usually pretty easy to pick them up with minimal chasing. This assumes it gets dark down there. If you have security lights or street lights this might not work really well. A fishing net may come in handy.
Good luck! Many of us go through this every year, you can manage it.
