Find a cab that you like the sound of, and get two. Or four.
Anybody talking about “a fifteen for the lows, “etc. is barking up the wrong tree.
You are not an audio engineer or an electrical engineer, but, luckily, all of the hard design work has been done for you. Mixing cabs, amps, etc. is usually a recipe for mud at some point.
Versatility comes from repeatability and consistency. Go for a small footprint, maximum height, and an array of matched speakers. When you do that, you increase the chances that every single person in the room is hearing the same thing (if your amp is carrying the room). The tonal balance that you seek to project needs to be happening before the signal reaches the speakers. It starts with how you hit the strings, and is mainly dictated by the pickups and the amp. The speakers are merely a vehicle- and mismatched speakers are an inefficient vehicle, so yes- you definitely want another 210 if you can get the same (or very similar) model.
Yes, the SM 12 is incredibly clean, accurate, and powerful enough to destroy most commercially available drivers, especially if you plan on using effects.
Mismatching cabs with an amp like that will result in audio Darwinism- the stronger cab will survive, the weaker one will go extinct (ask me how I know this

) Save yourself the headache (and $$$) and refer to the first sentence in this post.
Good luck, and rock on!