Retroactive Integration?

[Sapphire Sebright]

I can try to isolate them by dividing the coop with a separation, but will that make fully integrating them possible?

If the separation is something like chicken wire, then that is basically the, "see but not touch," form of integration, which does work.

 

[Backyard Dacks]

For how long should they be divided? What's next?

 

[Sapphire Sebright]

For how long should they be divided? What's next?

I think that the separation goes for a week or two, and then you let them mingle together (in a large space with a bunch of hiding spots, just in case) for a while. I'd separate for a week or two, let them mingle for a day, then separate them for the night and let them out together again in the morning. Do this daytime mingling/nighttime separation for a couple of days, and then try leaving them together overnight and see how they're doing.

 

[Backyard Dacks]

That's not very feasible, for various reasons. Is there a shortcut, like showing the vets preferential treatment?

 

[aart]

Is there a shortcut, like showing the vets preferential treatment?

Not really.
Posting pics of your coop and run might help garner viable solutions.

Here's some tips about....
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.
Good ideas for hiding places:
https://www.backyardchickens.com/threads/a-cluttered-run.1323792/