I have 5 2 year old hens in one coop and I have 6 9 week old hens in a coop right next to them. They always see each other but when is it a good idea to let them free range together outside of their runs?
How long have they been next to/able to see each other? If more than a week, I'd try supervised free-ranging. Have either/both groups been free-ranging already, or will this be a new thing?
If they've been next to each other for a couple weeks or so, I'd try ranging them late in the day(an hour before roost time) to start out. Here's some basics that might help.... Integration Basics: It's all about territory and resources(space/food/water). Existing birds will almost always attack new ones to defend their resources. Understanding chicken behaviors is essential to integrating new birds into your flock. Confine new birds within sight but physically segregated from older/existing birds for several weeks, so they can see and get used to each other but not physically interact. In adjacent runs, spread scratch grains along the dividing mesh, best if mesh is just big enough for birds to stick their head thru, so they get used to eating together. The more space, the better. Birds will peck to establish dominance, the pecked bird needs space to get away. As long as there's no copious blood drawn and/or new bird is not trapped/pinned down and beaten unmercilessly, let them work it out. Every time you interfere or remove new birds, they'll have to start the pecking order thing all over again. Multiple feed/water stations. Dominance issues are most often carried out over sustenance, more stations lessens the frequency of that issue. Places for the new birds to hide 'out of line of sight'(but not a dead end trap) and/or up and away from any bully birds. Roosts, pallets or boards leaned up against walls or up on concrete blocks, old chairs tables, branches, logs, stumps out in the run can really help. Lots of diversion and places to 'hide' instead of bare wide open run.
Do you want them all in one of the set ups eventually? If so, I too would let them out together a couple of times letting them return to there homes at night. And then I would flip the set ups, put the old where the young are, and vice versa. This allows the groups to see the others in there spot, and allows the newcomers to explore the area in their terms without being chased. Or if you want everyone in the old ones set up, and the little's set up is not big enough for them, then wait until noon, let the old ones out, and lock them out, put the new ones in the set up. Let the old ones back in at close to dark. The urge to roost will be strong, and that will limit the urge to fight. I really think that if they are in the see no touch situation, and you let the new explore the set up alone without the old ones, you will have nearly no problems. AS LONG AS YOU truly do have enough space. Mrs K