Adopting a second cat is a wonderful idea! Temperament of the new cat is MUCH more important than age. You would not want to get a cat with a very dominant personality. (and females can be VERY dominant). Your best bet is NOT a very young kitten. 6 months or older are usually accepted by the residents quite well. The babies (2 to 6 months) are usually too active for a resident adult, so unless you adopt TWO kittens, look for the older ones.
Best of luck, there are many deserving homeless cats just waiting for a chance at a forever home.