More than likely they will catch onto the roost on their own. I read that the farm they came from they were just in an open (indoor) space without roosts etc. So they really don't know what the roost is for. As they mature they will probably figure it out.
I usually introduce a "junior roost" to my peeps at 3 weeks. They are bouncing around and like to be above the others, even if the roost is only 3 inches above the ground at first. I also bring them outside at that age and they are very savvy with height and foraging by the time they are going into the coop with the big girls by 6-8 weeks.
As for "messing" in the nesting box, they will more and more check out their surroundings and sit in the box, and yes, poo in the box. Keep it clean, but as they get the urge to start nesting they will act like they are laying eggs without anything happening. I always like to have my new girls with my old girls quite young ( I used to wait until about 16+ weeks before mixing the new and old). I have found that this extra time allows the young girls to spend time with the old ones and watching their nesting behavior, so when the time comes they emulate it and then they lay where I want them to.
But....don't be surprised if when the time comes they decide to make a small space under a cozy bush and lay their eggs there. Once they lay you may have to keep them cooped up to get them thinking nesting box instead of under a bush.
My first flock came up to lay at different ages (I had a mixed flock with 6 difference breeds) and the first to lay kept wandering through my makeshift hoop/house greenhouse. I had a table in there with plants on it, but at night it was getting cool, so I put the plants under the table and covered the whole thing with old sheets and an old tablecloth (a pink tablecloth). One hen (upper pecking order) started nesting under the tablecloth, but she didn't lay any eggs yet. Then the others started coming in and peeking around the tablecloth at her sitting on the nest, peep show style!!
Anyway, they ALL started taking turns behind the tablecloth!! Then they started laying eggs, behind the tablecloth!! I tried to get them in the coop, but they wanted the pink tablecloth. They won. I crawled into the coop with a hammer and put nails at the top of the wall in the back where the lovely nesting boxes were. Then I took clothespins and pinned the pink tablecloth up making a curtain over the back corner and added more straw to the corner. My husband put in a cute little door in the back, so I could just go around, open up and gather the eggs. Yup.....they all started (well, all but one cheeky hen who NEVER laid in the coop) to take their turns behind the pink curtain to lay their eggs. Even when I added more hens the following year, they learned from the older ones that the magic curtain was the "place" to lay the eggs!! Sometimes you just have to watch them and give in to their wants over yours.
As for pecking order, that will iron itself out as well. It isn't always size, but attitude that wins in the end. Since the little one was there first, she may let the others know she is boss, but if the larger one decides she's willing to fight it out, you may even see some "rooster" behavior. One sitting on top of the other, beak to beak confrontation with jumping in the air and lashing out with their feet etc. They will decide for themselves, then they will settle into their flock and that will be that.
If you even decide to add more, or to replace a new one you will not be able to just toss it in with the others. You will need to keep the new one confined BUT in sight of the others, like building a 4x4 cage that can be placed in the run with the others. This is so they can see the new girl, but not fight. In a few days to a week they have had conversations and gotten to know the new one so they can be put together. They will still peck, but not usually as nasty as if you just tossed the new one into their space.
Aaaaahhhh chickens, and people think they are stupid!!
Tami
