I was looking at this site:
Cavalier King Charles Spaniel Club - USA - Breed FAQ
How are Cavaliers with other pets in the household?
Cavaliers are the ultimate groupies and are usually delighted to have the company of cats and dogs of any size. If you have a large dog, you will need to watch your Cavalier puppy carefully while it is small. Because Cavaliers are spaniels with a strong sporting instinct, they should be watched closely around birds and other small animals as well. Many Cavalier owners report that they have trained their dogs to live happily with hamsters, gerbils, rabbits, and the like. (In households where no one is home during the day, the companionship of another dog or a cat is highly recommended.)