Hmm, very puzzling...I'm not sure what to tell you...You said you are afraid that the cat will hurt the dog? I would be more concerned that the dog will hurt the cat...

My dog Duncan used to attack Jack, my roommate's golden retriever whenever another dog walked down the street. Not sure why, I guess it's some kind of redirection of anger or anxiety. Maybe the movement you make in the night startles Maggy and she takes it out on the cat? I would definitely try to figure out the root of the problem and stop the behavior before you bring a new animal into the picture though.

Good luck! Maybe someone else here will be able to help you more?