I have a dog and cat that are best of friends. My dog Koko is large at 100 lbs and Rascal my cat less than 10. I have found with MY past experience that smaller dogs are more aggressive toward other dogs or cats and are less adaptive. The real key is the cat and it's reaction to having a dog in the house. When Rascal and Koko were put together Rascal was fearless so they adapted very quickly. Koko is a Golden/Lab mix, and both breeds are easy going and not proned to Jealousy. Both Koko and Rascal were grown when they were introduced. You can see pictures of them together in my albums in my profile.