The last time I reintroduced a hen with tinies, I put an upside down crate in their box (inside the main coop/run) before turning them loose. I had holes cut into it on each side, different sizes, but all too small for the older chickens to get in. Once the babies were used to playing and hiding under the crate (a little over a week,) I put the whole shebang into the big coop. It worked like a charm. All four tinies are now healthy adults, fully integrated into the flock. I'm about ready to put one of them back in with her lone baby ... using the same crate.
Good luck with Rusty. They're super sweet little birds. My "pet" roos will walk on leash, so they do classroom visits when I do "Breed Conservation" talks at our local schools and 4H meetings. It's pretty cool! Make sure you check out the website for Livestock Conservancy's website. There is a really nice breed profile there, and you'll be surprised at how many other familiar breeds are in trouble ... from chickens & turkeys to Giant horses. It's scary!
https://livestockconservancy.org/index.php/heritage/internal/conservation-priority-list