Yes, ferrets were origonally breed to kill rabbits & rats. A ferret can easily kill a cat and it has been know to happen. And just like any animal they can turn at any time and go after anything that they may have been friends with before.

Ferrets have never ever been a wild animal, they descend from from other wild animals (minks, etc..) but have always been a "man made" animal.

I have 3 ferrets (today will be 4) and 8 rats, they all get along but of course I do not ever leave them alone & when they are together not for long. But especially Chaos loves the rats, he will climb right in the rat cage & hang out with them, they all cuddle up together & groom each other. I also have a dog that would love the ferrets as a treat but again they are never ever together unsupervised.
I do know quite a few people that have ferrets & rabbits & cats & they all get along great.
It all boils down to being responsible, taking precautions & having common sense.