We have 5 2 yr old rescue Warrens and one austrolorp rooster, they are all very chilled and friendly and I think quite healthy. They live in a large enclosed run with deep litter, they have a house which I believe has some more capacity.
Someone nearby is selling a breed i'm interested in - black orpingtons, which are closely related to austrolorps - 6 week old hens.
These are our first chickens so I've never dealt with little ones, or adding to a flock.
I'm thinking of getting 3 of the 6 wk old chicks, and borrowing an ark so that i could put them in the ark, inside the run, for a few days, and then just take the ark away and hope that they get on with the others. Is this ok? Do 6 week olds need any special care?

More than three would be a bit better; but given the time of year, they should be fine with basic care. They should be fully (or mostly) feathered out with their juvenile feathers by eight weeks... I would be concerned with introducing motherless chicks into a flock before week 12 (at a minimum); though my preference is closer to 16 weeks. But that can depend on the temperment of the older birds {shrug}... The best are chicks with a broody hen: true or adopted mother. But they are too old for that, and you would need an accepting broody.

Thank you. Yeah, we don't have a broody and I'm not sure if these were mother-reared or incubator/hand reared in any case.
Our flock is 5 older warrens who are very calm and tame. There is only one who was a bit pushy - and then I *think* only with one of the others. When we got the Austrolorp rooster that hen has calmed down. THe rooster has been incredibly calm and friendly with both the hens and us from day one. I'm not sure how old he is but not that old I think. The six of them as a group are really chilled, I notice very little henpecking or attitude in the flock. So I am hoping for the best.

Give them an area that only they can get into. Such as hardwire screening with a 10cm square opening or two in it.

I combined a flock of 17 one-year old hens (and two piglets) with a flock of 100 6week old birds and they all seem to be doing great. I planned it out so that the younger birds were established in sight but out of reach from the older birds for about 2 weeks before mixing them up. They were in their own paddocks next to each other, separated by two rows of electric poultry netting. I would feed them near the fences so they could get used to seeing and hearing one another at chow time. In this case the chicks were getting a little starter feed scattered on the ground so that they could learn to forage better. And since you can't give candy to one flock and not the other, I tossed a little to the older flock as well. I think this made them friends.
Then one night I moved the electric netting so that the paddocks were combined and enlarged. Everyone woke up and came out of their respective coops/pens and went to scratch around. There were a few little battles but nothing to be concerned about. If something gets rowdy between the chickens, the pigs usually go to investigate and break it up. I think the fact that there were too many young birds to fight, made the older hens just kinda give into the idea of sharing the space. They still stay in separate coops though. At least til the roosters go off to freezer camp.

In your case I think I would be concerned about two things. Make sure the rooster isn't too rough in establishing himself with the new hens. They can be pretty brutal. I butchered our rooster before the new flock went outside to avoid giving him little targets to beat up. That also messes up the pecking order for the older hens so that when they got integrated with the younger birds, they really didn't have a "boss".

The other thing I would think about is a quarantine for the new birds. This way you don't accidentally bring illness into your flock. This is more of a judgment call but I like to er on the side of caution.

Best wishes

I did the same a few weeks back. I had to spilit my coop with chicken wire because the pecking got pretty brutal. They were fine outside, but inside there were hostilities. After two weeks I was able to remove the barrier and they all get along now.

Thanks again.

We built a little ark and got the three new ones today. We brought them into the run, shut the gate, and tried to get the from the crate into the ark, but they escaped and went running around the run. The others were surprised and curious, but didn't attack them - but of course we were in there trying to catch them. One of them went into the big hen house, and was in there for a good five minutes while I was catching the other two, and no one bothered her. Finally got all three into the ark. Observing them for the afternoon, the big ones seem no more than marginally interested in them - they occassionally stroll over to peer at them and try to steal some of the lettuce from inside the ark, but mostly they don't seem fussed. I'm hoping this bodes well for the future but of course once the ark is gone, all bets are off. Since they're Orpingtons they're not too tiny for 8 weeks, but our rooster is huge. So I'll give them at least a week in there and then see what happens - once we're ready to let them out, I may try making a doorway into the ark that's only big enough to let the chicks in, so that they have a respite if they need it.