I strongly recommend that you do not get a kitten. That would be like giving your 80 year old grandmother a two-year-old child to live with. She would go bonkers quickly.
If you feel you need to adopt another cat, it should be an older cat (around 8 to 15 years) with a calm and friendly temperament. I can't say strongly enough how important it is to adopt an older cat (or none at all).
If you do decide to adopt, make sure you do a careful introduction of the two cats using the method below. Whatever you do, please do not just throw the cats together the first day you bring home the new cat.
First, put the new kitty in her own room. She will need to stay there for a couple of weeks, so make sure it is comfortable for her with litter box, water, and food. This will allow your old cats to get used to the sounds and smells of a new cat without having their territory invaded.
After a couple of days, start trading scents between the cats. Rub the new cat (especially around the cheeks) with a slightly damp towel and then go rub the old cats with the same towel (and vice versa). This way they will associate the scent of the new cat with good things (being rubbed and getting attention). Do this several times daily.
After one week, lock up your old cats and let the new cat out for the night or for a few hours. Do not yet allow the new and old cats to be together. Do this for one week.
By the end of week two, your old cats are going to be really curious about what is going on with the cat behind the door. You can try bringing the new cat out with the old cats for brief visits. As soon as the new cat comes out, give them all some tuna or some other really yummy treat. This way they associate great things with the other cat. Put the new cat back in her room after a brief period of time. Make sure you keep any aggression from occurring (GENTLY toss a pillow at the aggressor).
Some people suggest putting a dab of vanilla extract under each cat's chin and at the base of each cat's tail (on top, not on their bottom). This way both cats smell the same....since cats are so scent driven, they are less threatened when the other cat smells the same way they do!
Gradually increase the amount of time the cats are together. I recommend keeping new kitty in her room over night for at least a third week. When the cats are out together, make sure you give lots of attention to the old cats so they know that they are the top kitties.
Expect lots of hissing and perhaps a swat here and there. But...this should keep any real aggression from occurring. The hissing will stop once they establish their hierarchy and get used to each other.
Most importantly....have lots of patience and take things sloooooowly!