When new kitten/cat arrives to a new home, where thers is another 1-2 cats already live - I would've advise "slow introduction routine". Resident cats are normally more or less territoril; New cat has a strange smells and strange look; I've hear that cats have no size perception - so it doesnot matter weather it is a young kitten or adult arrives in a new home - it coud be threatening for exosting cats. New kitten should be places in a separate quiet room (with litter tray, food and water) to adapt new noises and smells. Sometimes its takes up to a few days for them to get Introduced. Than they all will smell each other under the door, may be one could exchange blankets for them to sniff each other. In a couple of days lock/hide older cats for a while and let new kitten to explore the house. When they will start to play "footsy" under the door - its a sign that they are getting much friendlier to each other.

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