Sure you can add them together. They will fuss for a few days until they establish the peck order. Just remember to quarantine any new chickens you get to make sure they aren't diseased, or carrying parasites. I have also heard that adding them at night eases the adjustment, but I've never tried it. I imagine they still carry on some. Its just natural for them to establish that peck order, but I've never had any serious injuries from hens. Roosters, of course, can be a bit more aggresive with one another, but in a big enough flock they will work things out to as long as they aren't too gamey.