When introducing a new pet to an already established resident animal, the less threatening the newcomer is, the better the chances he will be accepted. To animals, non-threatening means younger, smaller, and usually a different sex. Chances that the two would ever get along are slim. Keep in mind, however, that a youngster can be very stressful on an elderly pet, since animals, as they age, become increasingly less able to cope with change. Animals also tend to get along better when they are all spayed and neutered.
Timing is also important.
Since introducing a new animal is stressful to the residents, it makes sense not to do it at a time when they are already under stress-when they have just moved to a new house or apartment, for instance, or when one is recovering from an illness or injury. For a successful match, choose a younger and smaller animal of the opposite sex. Usually, supervision is very important, as a resident dog can easily hurt a new kitten, or resident cats, frightened by a playful puppy, can scratch the pup's nose or eyes. It's safer to get a slightly older kitten, perhaps six or seven months, if there is already a resident dog, since he will be better able to defend and protect itself. You can help dogs start off on the right paw together if you introduce them on neutral territory, perhaps in a park, instead of at home, so that your resident dog first regards the stranger as a possible friend rather than as a threat to his territory and dominance.