Hi MaryPhill,
I think KOPsarah's redirecting prey drive to fetching a ball a brilliant idea!
In addition to that, I would recommend you train your dog "Leave it" or "Drop it" command.
I have a border collie/cattle dog mix that is very obsessed with squirrels and Holly actually caught a squirrel when walking off-leash on a bike path twice. As soon as she caught the squirrel in her mouth I shouted "Leave it!" and she let the squirrel go both times. She is almost 100% trained with those commands. Those are very handy commands when you drop a piece of chocolate, onion, medicine pills or anything your dog is not supposed to eat on the floor.
As KOPsarah mentioned dogs can learn to accept cats as part of their packs as well. We introduced an adult cat to our 3 dogs (3 border collie mixes) about an year ago and they are now all friends so I don't need to supervise them any longer. We kept telling our dogs to "Be nice" or "Be gentle" while we hold the cat in our arms. In fact the cat now thinks he is a dog sometimes. He even goes for a walk with our dogs.
I am now fostering 2 baby kittens. I still have to watch my dogs when the cats are loose with them since they are so tiny but I already see a difference in our dogs' attitude towards those kittens. They don't see those kittens as prey/toys anymore and they treat them with some kind of gentle care.
Holly was so scared of men when we first adopted her 5 yrs ago. She was abused by men with sticks or brooms. We tried to desensitize her gradually using treats and comforting her every time she sees a stranger. She used to bark at every man in the dog park. I asked each of them to give some treats to her (she was and still is so food motivated since she was a skinny stray when she was found) while I was standing next to him and telling her he is just fine. She has grown to be a very confident dog.
Everything boils down to training and acclimation. It takes time and a great deal of efforts and consistency but I am sure you will be able to train your Rottie mix.
Good luck
