Madpiano, we got Spike and Oz almost a year apart, and they're the best of friends. As long as you take pains to introduce them carefully (and this forum is full of advice on that topic!), I don't think you'd have too much trouble. But get the tomcat neutered! (He will be territorial, he will spray, and if you get a female cat ... well, you get kittens.)
In my experience, female cats seem to mature faster. Now, that might be social bias, as I know human females are considered to mature faster, and I could just be projecting that knowledge onto my cats. However, when we lived with our former roommates, they had a female calico and we had a male tabby, and while they were fairly close in age, the calico certainly seemed to grow up first!
I have heard that it can sometimes be hard to introduce female cats--to another female cat, or to a male cat, it doesn't seem to matter--whereas male cats have an easier time of it. I'm not sure if this is true (I would have expected more dominance issues between male cats than between female ones), but this is what I've been told.