I would start my mingling with supervision from you during the day. Yes they could seriously hurt the new ones without a proper introduction. It might take more than a week, it depends on your birds personalities. Introducing new members takes some time, don't rush it.I have four chickens, and I just got two more yesterday ! I am seperating the two new girls from the oldies for a week! When it's time to put the two new girls in with the others at night , do I have to worry about them killing the two new girls in the morning? Will I have to get up before they do and wait to make sure everything will work out? Or will the old girls already be used to the two new ones?