Testing for Tolerance
If you wish to see if your Chihuahua can get along with a cat or vise-versa, the best thing to do is test their interaction. While it most likely will be that the dog does not tolerate the cat, in the case of a Chihuahua, care must be taken. Since many cats are quite larger than this breed, even if a dog begins a fight, the cat may end it.
Cats are able to leap down on a Chihuahua from above and this can cause great injury to the dog. In addition, the cat's claws (if not de-clawed) can cause severe injury to a dog that has little protection in way of the coat.
Before creating a multi-pet household, you'll want to see if see if two types of pets will get along. Whether you have a Chihuahua or a feline, do you best to find a friend who can bring over the type of animal that you wish to use to test the interaction.
For example, if you have a cat and wish to add a Chihuahua to your family, try to find a friend, family member, neighbor or co-worker who would be willing to bring their dog over for an experiment. Stress the fact that this will be 100% controlled and no danger will come to either animal.
When you have the two animals in the same room, your behavior must be calm and casual. If you tense up, sit and stare to watch what happens with great intensity and are worried and anxious, a Chihuahua can pick up on those feelings. You do not want your actions to silently be telling a dog that something is wrong or that danger is eminent.