My old set up years ago was in the barn. My cats were indoor/outdoor cats and spent a lot of time catching mice. Never attacked a chicken. I think a flogging even by a hen would deter most cats from giving it a go.
x2 on the getting her vetted. My uncle learned the hard way that rabies can be an issue in barn cats.
I would think having a mouser around would be a good thing. Just be safe and get her vetted/spayed.
I would also think that if you are planning to hatch chicks out that would be a concern.