I've never heard of DLC that aren't compatible with each other, seems like an amazingly bad idea. As for being able to turn them on and off, I'd love to see this with both DLC and Expansions, but I recognize that this isn't always possible because of the way these sometimes change the code of the base game.

Most DLC will probably be stuff that directly enhances the game like planet/star types/textures, ship parts, more event types, more nebula/asteroid types, new factions, etc. So those will all be compatible.

With the scenario dlc, like the snathi, it depends on how they do it. They could just make it a custom map, that is the same each time you play it, which has the benefit of easier scripting events/encounters (which i presume is how they will do the campaign). Or they could just make it a special faction/AI script that uses normal map generation, in which case all the other dlc would also work.