Getting a Kitten

My family is thinking of getting a kitten, but we're woried about our other cats, especially one called Millie. We think that Millie may feel a bit challenged by this next addition to the family, but she isnt vicious or anything but we think maybe the kitten would be vicious against her so we are not sure how we would handle the situation. Millie can be very shy, and because our other cat is usually out a lot and only comes in to eat or have a nap, she doesn't feel threatened by the other one. Anyway if we get this kitten she may just not come from upstairs and just hide away there. Has anyone else had this problem and how do you handle it.
Should I get another one or not?

We have an older cat and we recently got two kittens.. At first the older one seemed a bit hostile about the 2 kittens who were very inquisitve about him.. But he's starting to accept them :_)

Thank you x
Now all I need to do is persuade my mum. She hates cats but the rest of the family really wants one. And we need to wait until everyone is the right mood because of family problems at the moment, everyone is rushed off their feet. And we need to find a place to buy one.
Ugh. Such hard work!