Yes, with rescues you never know. It's good you've kept her separate from the rest of the flock, so she doesn't contaminate a healthy flock with anything. You should keep her quarantined for at least 21 days. I typically dust, and worm newcomers before introducing them to an established flock. I'm not a big advocate of DE. While it's touted as natural, the assumption is that it's also safe, and effective too. That's simply not true. In addition, there are far too many that will attest to the fact that when their chickens had mites, the DE failed. When facing an infestation, it's best to use a product known for it's efficiency. This provides quicker relief to the chicken too, which should be the goal.