Cats are territorial. If you are adding a new cat to your household, stick to a kitten. It's much harder to get older cats adjusted to one another. An adult cat often sees a kitten as someone he can discipline and be dominant over, so fewer problems occur.
Preparing Your Adult Cat for a New Addition
Before bringing a kitten home, it's important to familiarize your older cats with the kitten's scent. Go to the home or shelter where the kitten is located and rub a blanket or stuffed animal on the kitten to capture his scent. Leave this blanket or stuffed toy in an area that your cat frequents.
Visit a Veterinarian
It's also important to have an adult cat health exam performed prior to bringing in the kitten. Many kittens will have had their first vaccines, but if the kitten has fleas or worms, you'll want to protect your cat by having him on preventative medications.
You do not want to expose your old cat to symptoms you are unaware that your kitten is showing. Some diseases are obvious, but others are less apparent. Make sure your older cat's vaccinations are up to date, including Feline Leukemia and respiratory diseases. If your kitten does show signs of these illnesses later on, your older cat will benefit from the advanced protection.
When You Bring the Kitten Home
On the day you bring your kitten home, plan to be around for that entire day. Spend time bonding with the new kitten, but also make sure you are giving your adult pet equal time to prevent jealousy. Feeding both pets treats at the same time is a great way to show both pets that they will receive equal attention.
Elderly cat behavior is hard to predict. Some cats willingly accept the new kitten without issue, others want time to acclimate. It's helpful to keep the kitten in a cat carrier for half an hour to allow your cat time to get used to the kitten's scent.
Your kitten is going to be curious and will head right for older cats. Expect to hear some hissing and your adult cat will probably run and hide. Limit the kitten's activity to one area of the house so that your older cat has kitten-free areas. Let your cat come and go as he pleases as he adjusts to the new pet. Avoid punishing the cat for hissing or swatting unless teeth or claws are used. It's common elderly cat behavior to swat away a kitten, but rarely will he extend claws.
If you must go out, place the kitten in a separate room with the door closed. Do not allow your adult cat and kitten to remain together unsupervised for a few days. When you notice both cats no longer hissing at one another, you're safe to let them both roam free throughout the entire house. Some squabbling may occur from time to time, but this is often playful behavior and nothing to worry about.