I say go for it. The younger your current cat is the easier it may be for it to welcome a newcomer. The older he/she gets the harder this may be on you and your cat. This will also give your kitten a play mate for when your at work or just out of the house. I started out with two sisters so it made it easier for me, but when I'm at work they play and entertain each other and it makes me feel better to know that neither one of them is alone. I later added a third when they were a little over a year old and there was a little tension at first. I did it while I was on vacation and could be there with them to make sure they
didn't gang up on the little fellow. Also consider this, if you choose a kitten of the opposite sex please plan on spaying/nuetering to help prevent over population. Two can quickly turn into 6 and the pattern keeps going on. Plus it is
healthier for the cats.

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"As we all know, cats now rule the world."
--John R.F. Breen