I found some really helpful tips on how to introduce a new cat to your home if you already have an established cat. If the cats are spayed or neutered, chances of them not getting along are greatly reduced.
With an older cat, the new cat or kitten should be younger. I would suggest at least a 4-5 month old kitten (no younger, please). The new kitten will have plenty to cope with and a bit of age makes adjusting to its new life much easier. – Norm
Follow the link below to learn about the techniques:
http://en.allexperts.com/q/Cats-1606/MALE-FEMALE-CATS.htm
Image:
http://commons.wikimedia.org/wiki/File:Abessinier_sorrel.jpg
You may also like:
Rescue Tuxedo Kitty’s First Thanksgiving 
Cat Watching Hockey 
Five Playful Kittens 
Don’t Worry, My Baby. Cutest Cats Moments 
Inseparable Kitty Friends: Then & Now 
Orphan Kitten and Puppy: Now An Unconditional Family 
Love Between Two Redheads 
Kitten Sleeping In Awkward Position
[...] for the top cat title in the house. All these factors can cause a range of stress to a cat. See How to Introduce a New cat to an Established cat for more [...]