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: