Some say to let them fight it out, but I don't think it's right to do it that way. You don't want to let one pick on the other, this can cause stress, which can damage their health. Cats usually get along with ferrets after they get used to them, but as with dogs, I would never allow unsupervised play, simply for protection of all of them. Under NO circumstance should you allow them out when you have birds or other small animals loose, ferrets are extremely fast. 1st few nights, if the fuzzie is very young, can be very scary for them.
Ferrets are so small that they can become critical quickly.
Ask other people, get in touch with a shelter that deals with ferrets. Ask your vet what they know about ferrets. See how your vet handles the fuzzie. I quit going to a particular vet because I didn't like the way they held one of mine to examine him. They didn't hurt Chuckles, but you don't have to scruff a ferret to examine it.