Cats are territorial, some more than others, and in most cases, do not initially take well to “intruders” in their territory, not even to small kittens. By doing a slow integration procedure, with gradual amounts of exposure over a whole lot of days or even weeks, building up to full-time mingling, things will usually work out fine.
Here are the steps:
1) Isolation: Isolate the new cat completely for a few days.
2) Scent Familiarization: Scent familiarization with items, such as towels rubbed on each cat, then left in the area of the other cat.
3) Room Swapping: Switching the cats' places for a while every day, with no physical contact between them to allow even more scent familiarization.
4) Visual Familiarization: Seeing each other with no physical contact, such as from a slightly cracked opened door or with one of the cats in a carrier or cage for a few minutes only.
5) Developing Positive Associations: Start developing positive associations with no physical contact, such as by feeding the cats on opposite sides of a door when cracked opened and even when it is shut.
6) Short Supervised Visits: Brief, highly supervised visits paired with food or treats.
7) Separation at Signs of Hostilities: Separate the cats at signs of hostilities, or great fear. Learn cat body language to tell when a problem is starting.
8) Longer, Carefully Supervised Visits: Mingling under very careful supervision for up to a few hours, before total separation again. Adjust the length of time depending upon whether there are any signs of hostilities or stress, but no matter how well things seem, do not yet permit 24/7 free mingling.
9) Limited Free Mingling: Free mingling except when the people are not home or asleep, so quick intervention can be done if needed.
10) Free Mingling Full Time: The cats are let together freely all the time when all the above steps have gone well. Backtrack to the limited mingling if the cats weren't quite ready to be together all the time.
Please read the following article for more details and help:
http://wvcats.com/integrating_kittens_with_cats.htm