Spend some time with them when you add her back. Sometimes a wire dog crate with food and water can be useful if one is picked on, but I would spend more time with them to see who is doing what. Pinless peepers are something to consider using on a particular bully.
I wouldn’t keep her separated unless she is being pecked. It shouldn’t take long to determine if this is caked on droppings, a scab from an injury, or dried drainage. There are too many problems trying to reintegrate a chickens that has been separated, so best to keep her with the others to...