I have 6 cats and they all get along. The thing that is most important is the introduction. You'll need to isolate the new kitty in spare room. Stack baby gates in the doorway so they can see and smell each other. Feed them near each other on opposite sides of the gates. They'll associate it with a pleasant experience. It'll take a while, but you'll be surprized at how they'll adapt to each other.

That's great that you're thinking about another cat. Only cats can sometimes get lonely. Good luck!