It sounds to me like you've got the perfect combo. From my experience, cats will have a little hissy fit when you introduce a new cat into their house, but the chances of them eventually getting along are much better if they are different ages and different sexes.
I may not do this after all, after thinking more and more about it. It's best if the kitten's owners either give their cat more time to get used to the kitten, or take it to a no-kill shelter to give it an opportunity to be the only cat in a new home. And I can't bear the thought of giving it away once I take it, and then it doesn't work out.