A good time to introduce the puppy to the household is when the kitten is about six or seven months old and better able to defend itself and teach the puppy some manners. 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.
Since territorial and dominance behaviors are affected by hormones, spaying and neutering naturally reduce these aggressive impulses. 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. Any disruption in their routine is stressful, so it's wise not to add the stress of a new animal at the same time a cat may be feeling stressed from another cause. The decision is made or perhaps it was made for you when those big brown eyes looked into yours.