Kay,
I also know how much you love Sim and Nala, and I have both dogs and a cat, and they love each other, but the cat was here first lol so I can't really help you on that part.

I want to post some great tips that really helped a friend of my moms, who introduced a new cat to her 7 year old German Shepherd.

If your dog does not already know the commands "sit, down, come, and stay" you should begin working on them. Little tidbits of food increase your dog's motivation to perform, which will be necessary in the presence of such a strong distraction as a new cat! Even if your dog already knows the commands, work with obeying command in return for a tidbit.

* After the animals have become comfortable eating on either side of the door(not sure what that means), and have been exposed to each other's scents, you can attempt a face to face introduction in a controlled manner. Put your dog's leash on and command him to either "sit" or "down and stay" using food tidbits, if needed. Have another family member enter the room and quietly sit down with the cat on his/her lap. The cat should also be offered some special tidbits. At first, the cat and dog should be on OPPOSITE sides of the room. Repeat this step several times until both the cat and dog are tolerating each other without fear, aggression, or other uncontrollable behavior.

* Next, move the animals a little closer together, with the dog still on leash and the cat gently held in a lap. If the cat does not like to be held, you can use a wire crate or carrier instead. If the dog gets up from its "stay" position, it should be firmly repositioned and praised and rewarded for obeying. If the cat becomes frightened, increase the distance between the animals and progress more slowly. Eventually, the animals should be brought close enough together to allow them to investigate each other.

* Although your dog must be taught that chasing or being rough with the cat is unacceptable behaivior, your dog must also be taught how to behave appropriately, and be rewarded for doing so. (e.g. sitting, coming when called, or lying down in return for a tidbit) If your dog is always punished whenever the cat is around, and never has "good things" happen in the cat's presence, your dog may redirect aggression toward the cat.

* You may want to keep your dog on a leash and with you when the cat is free in the house during the introduction process. Be sure that you cat has an escape route, and a place to hide. Keep the dog and cat separated when you aren't home until you are certain the cat will be safe.

PRECAUTIONS: Dogs love to eat cat food because it is very high in protein, and therefore very tasty. You should keep the cat food out of the dog's reach (in a closet, on a high shelf, etc.) Why dogs like to "raid the litterbox" is not well understood, but eating cat feces is a relatively common behavior. Although there are no health hazards to the dog from this habit, it is usually distasteful to owners. Unfortunately, attemps to keep the dog out of the litterbox by "booby trapping" it will also keep the cat away as well. Punishment after the fact will NOT change the dog's behavior. Probably the best solution is to place the litterbox where the dog cannot access it such as behind a baby gate, or in a closet with the door anchored open (from both sides) just wide enough for the cat. Another helpful hint is to buy a covered litterbox and turn it toward the wall with a space big enough for the kitty, but not the dog.

I hope that helps a bit

Good Luck, Kay!