How old are your younger ones, i see you said last year, mainly wondering if they are full grown? The few times we have added new hens they were nearing comparable size. There was definitely some fighting when food was found and the new girls stayed out of the way of the established ones, but after a week or so everyone was okay. It was definitely hard to watch them fight a little bit (and I really saw where the term pecking order came from!) but after a while they all settled in. I would watch them closely when they are introduced and break up any really bad fighting that looks potentially deadly. It sounds like you have a lot of room for them to run and hide too if they are feeling too picked on. The first time we added hens there was no fighting at all and we did the sneak them in at night idea. The second time we did the same thing but there was a bit of fighting. This time, like mentioned above we are doing what Patricia Foreman suggested and letting them see each other through a
fence for a while to try to get used to each other and then put the vicks on all of their necks at night so they all wake up thinking "hey we all the smell the same, we must be a flock". We're going to wait until the new ones are a bit older so that they can fend for themselves a little bit and i was going to try to just throw a lot of stuff for them to eat out in the paddock they will be in, hoping that they are too distracted with getting food to be too worried about fighting.