A lot of people ask, if I get another pet will it get along with my cat? The corollary to this question, if I get a cat will it get along with my existing pets, is also of interest to some folk. There is no simple answer to these two questions, but there are some facts to consider that might help forecast the results of such interspecies interactions: The species of the housemate you intend for your cat (or proposed cat)
The temperaments of the individuals to be mixed
The early and later experiences of the individuals to be mixed
Which species is incumbent
Our own ability to monitor and manage the situation
The environmental setup
While there can be some very harmonious marriages of species, sometimes the result of the mix can be damaging – or even lethal – to one or both animals.
Dogs and Cats
President Clinton found out that bringing a dog (Buddy) into the White House where there was already a cat (Socks) was not as easy as balancing the U.S. budget. The two fought like, well, dog and cat. But do all dogs and cats hate each other? The answer is no. The relationship between these traditionally acrimonious species can range from good friends, to indifferent, to positively hostile.
Dogs, by nature, are predators. Predators tend to chase rapidly moving and furry things smaller than they are, which is the job description of a cat. So, potentially there is a problem. But, dogs and cats, like humans, are not driven by nature alone. There is also a learned component to what they do.
For a dog and cat, the most important time for learning who your friends are is the so-called sensitive period that spans the first two to three months of life. A puppy that is raised with cats during this time, and experiences no adverse consequences of the interaction, will likely grow up to regard cats as benevolent domestic fixtures. The reverse is also true.
It may be slightly easier to introduce a new kitten to a resident dog than to introduce new puppies to a resident cat because of the highly territorial and antisocial nature of some cats. But you can also have your work cut out introducing kittens to a highly predatory species of dog. Both situations can sometimes be managed by proper chaperoning and protection of the most vulnerable species, and time can lead to mutual tolerance if not mutual admiration. If puppies and kittens are raised together, neither party should present a problem when integrating with the opposite species unless the incumbent is particularly mean.
Cats should not be introduced to a home with dogs that have chased and tried to kill cats. These dogs will probably find it difficult to see cats as anything other than prey, and even if they do not actually manage to catch the cat they can make his life pretty miserable. Likewise, a puppy may have to be protected from a territorial bully of a cat that has, by virtue of his prior experiences, a lifelong hatred of dogs. Sometimes a dog in such a situation will learn to avoid a dangerous, unequable cat. In other instances, the cat may spend his life in trepidation of the dog. Neither of these situations is desirable or reasonable and they should, if possible, be avoided by prevention or rehoming of one or other of the feuding parties. That's what happened to Socks.
If you are thinking of mixing species, ask whether they are predatory, aggressive, territorial, solitary, or gregarious. That will give you the genetic drift on what to expect. Then ask, how the species was raised, with whom, by whom, where, and when. Next you should probe for any information about prior interspecies interactions of the species in question (if that's not already moot).
Finally, if you are still up for it, insist on a trial marriage before you commit to the newcomer. Not every creature gets along but then again, some do. Sometimes you just have to try putting future housemates together to find out how they interact together. But be safe. Their lives are in your hands. With the correct early socialization some seemingly miraculous unions can be achieved, like cats that allow birds to perch on their heads, cats that allow mice to run all over their bodies, even when they're nursing (there's another generation of mouse friendly cats in the making!), and cats who allow themselves to be groomed by non-human primates. It takes all kinds to make a world, and all kinds of (sometimes unlikely) unions to make it a happy place.