The way I figure it, the "True Sith" really shouldn't have anything to do with the Sith as we know them. Meaning that we should have absolutely no idea about their origins. Kinda like the Rathkata (or however it's spelled), in the sense that they are a society of Force sensitives (I think that's a must)-one that "naturally" adopted the use of darkside powers (kinda like the drow, to give an example). You don't become a True Sith, or join them, you are one by birth or not at all. Maybe they never knew of the Republic and it's star systems. Maybe they learned of it from one of the "original" Sith.
I'm pretty sure that is the implication (that they aren't at all related to the Sith as we know them) made in Kotor 2. Sure, the idea of the Sith will exist as long as the Force exists, because it is a different way of using the Force. So the True Sith in my mind would be everything that Sith attempt to be without even trying (mainly meaning powerful in the Force, specifically with the darkside, and also aggressive in terms of concrete power).