Typically the lowest hen in your older flock will be the worst offender on the pecking. She already knows where she fits with the others but she will want the new ones to know where she fits to them. That's the one you have to watch. If she just pecks them on the head when they're walking by...
There's a lot of schools of thought on this one. Me personally- I just dump them all together and watch for 30 minutes for a serious problem. If no problem, I come back in a few hours and check again. Your older hens are going to boss around your young ones and let them know the pecking order...