Are you planning on adding a single hen or several? You allude to both. Adding a single hen is going to be rough on her and you. No two ways about it. Plan on it.
I just introduced a rescued hen who was the sole survivor of her abandoned flock. It's been seven days, and the flock is still chasing her and pecking her. I began by putting her in a large crate in the middle of the run. That was not the best thing as some little savage managed to bloody her comb through the wires of the cage.
Then I switched her to a safe pen, which was much better. But she began to get attached to being in her own little world. It wasn't moving the flock along toward true integration, so I began to leave the gate to her pen open after she has finished eating. That's another big problem with adding a single hen. They will be afraid of eating and may go hungry unless you provide a safe place for them to eat.
At night, roosting hasn't been the huge problem I feared. But she's still not confident of her right to roost where she wants, and is relegated to a low perch she hates. But generally, the others are too focused on roosting to give her much trouble.
If you have the choice and plan to add more than one hen later on, I would wait and include the single hen in a group of hens and add them all together. Grouping new chickens together will inspire more self confidence in them and make them less vulnerable to bullying.