I found a man on Craig's list who has 3 pullets he is selling. He had to buy 6 bit only needs 3. What are your thoughts on these? He says they are all female. How do I introduce her to new hens?
Mrs. K has covered some of the basics there, such as obstacles and hiding places and not interfering unless necessary, as it makes it worse.
Generally, for first introductions, you should keep them in a separate coop (rabbit hutch, run, whatever) for a week or so in full sight of your hen, if necessary put treats beside and in the cage so (with a wall between them) they do some communal feeding and spend time in one anothers' presences (if she tends to free range elsewhere and ignores them).
The more they interact vocally and with body language with the safety of a mesh barrier between them, the more likely their eventual introduction is to be peaceful or at least nowhere near as destructive as just instantly combining them can be. They can sort out who's alpha and who's not without direct conflict if they're just given a chance to eye off the competition first. But chances are at least one of them, the alpha, will fight your hen to start with.
Of course they may be --- quite possibly are --- carrying diseases and perhaps parasites that your hen has never been exposed to before, or maybe newer and different strains of the same diseases. So quarantine may be useful, and worming or whatever, prior to introduction. They can and often do look healthy despite carrying diseases and parasites though.
I'd think if a predator took your rooster she'd be quite distressed, it kind of does sound like a theft. I've had complete mutts stolen, nobody could have mistaken them for purebreds. It's happened to many people. I'm not sure of all the many potential reasons why anyone would do that, it is indeed baffling.
Best wishes.