Caveat: I haven't read Neptune's Brood.
There are a couple ways you could steal a world.
- You could infiltrate the government to the extent that there's no real resistance to your efforts from other politicians.
- You could change the values of the people so that they're all essentially in on your conspiracy, and the government has no choice but to bow to your demands.
- If you actually want to physically swipe a world, you should probably go for a small one. Planets are heavy. Like, really heavy.
All of these work better if the world is isolated when you start. You might have to arrange for that. Be subtle.
If you go with method #2, then yes, chances are that everyone would be part of your Society, though many would probably have the Solidarity CV at too low a level to learn the Criminal Profession.
When I initially came up with the Organized Crime Society, I imagined them as a group that spread across many different civilizations, such that you could rely on Solidarity from other members of your Society. I think it probably works better if there are many different cartels, some of them quite vast, others more local. Some of the big ones would undoubtedly control some of the smaller ones.
Does that help?