Good luck. Yes, extra coop great for see and don’t touch. Do this for 2 weeks, at least. Then, open up extra coop for a bit of integration…while you are there and watching. Do this for 3-7 days (depending on how it goes), keep watch during this phase. Next, put new girls out and into run for a few hours, can’t get into their old coop. Let them back in after a few hours. A few days may suffice. After this, all day integration, your choice to put them in coop at night or see if they go in all by themselves.
I’ve had SF before, from 3 sources, they don’t defend themselves. They sit and take the pecking order stuff that is dished out. Beautiful birds, but not a good fit for my mixed flock.
I’ve had Marans before, they tend to be assertive, might cause problems.
Put up a couple roosts in the run, this allows birds to get away from each other.
Have two feed and water stations, bc low ranking/new birds can be bullied away from food water, and eventually get sick and die. Pecking order is very much a real thing.
We’ve never integrated older birds, but have integrated hundreds of chicks. They get see/no touch until about 4 or 5 weeks. Then we raise up the covered portable fence we use, and put garden stakes around it to create a barrier to the hens. This way the chicks can go in/out at will to a protected area, but hens cannot. It takes time, and some years we have a real bully or two in the batch of hens, so we have to keep a good watch out for any bullying. One year was terrible, we put up several roosts in the run, which helped immensely. Chicks could get up and away from the hens. Roosts are only about 2 ft off the ground.