I have found that keeping the new comer in a seperate bathroom for a day or two helps. The older cats can check out the newbie through the door and it also gives the kitten a small place that isn't as large as your entire house. After a day, i just let the new guy out. It usually takes a week for everyone to start cuddling together and paying. You may also find that your kittens stick together and don't bother with the older cats.