I don't think you can get a much more thorough answer than what Jen gave. And I agree whole-heartedly with what she says.
I think it could be nice for your 9 month old to have a playmate. But please do get her vaccinated and tested for both feline leukemia and feline aids. And absolutely get a second litter box. If you have room even get 2 more. My youngest cat, Vixen is super anal about her boxes and she will only peepee in one and poopie in the other. If Lilith goes peepee in her poopie box or vice versa Vixen raises the roof. LOL

Let us know what you decide and how things turn out.