Hi welcome to the forum.I have no experience of introducing a new kitten but you may find this useful
Living Together - Introducing a New Cat
Hope this helps

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my 6 year old and two 5month olds are like that, they have been like that even since i brought the two kittens back when they were 2 months old, their just play fighting, after a while you will see they wil just tend to walk pass each other without doing anything but every so often they will give each other a slap, my kitten kept slapping the oldest around the head so the oldest slap back, and they lick and sniff each other most of the time as its their way of getting to know each other

i had a similar situation with a 7 month old and bringin in a new kitten, from what i gather its their way of establishing whos boss a few days - 2 weeks and theyll warm to eachother my 2 still wail when theyre play fighting but they love eachother really as theyre rarely away from eachother and theyll sleep together itll just be the playing that the smaller 1 will have to get used too

It just takes time. Don't worry too much about it. Give them both lots of fuss, especially your original cat. The link Buffie posted is an excellent resource for you to turn too if you're unsure on things.

Give us some pictures of the new Siam when you can.

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Two cats: Maia & Moth, Siamese. One dog: Harley, Lurcher.
RIP Lily my darling pirate cat.

I agree that the problem will be solved when the small kitten is growing up. The problem begins when a male cat who has been living in a house for many years meets another male cat... that's a real problem.

You could try a Feliway diffuser which is useful in creating a stressless atmosphere when there is tension amongst the household.

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