Okay. I'm pretty sure I'm coming down with a vengeful cold, but since I'm here...
There are five "levels" of Star Wars canon. G, T, C, S, and N, in that order.
G-Canon is George Lucas Canon. This is exclusively the six movies and anything from those six movies used in any other work. (Since you're all wondering, G-Canon is the latest version of the movies, since they are the most in line with Lucas's ever-changing "vision".)
T-Canon is Television Canon. This is for the original 2D Clone Wars series, The (current) Clone Wars series, and the future live-action series.
C-Canon is Continuity Canon. This is basically all the new, and a lot of the old, novels, games, comics, cartoons (Droids, Ewoks), non-theatrical films (like the Ewok movies), and all of that other good stuff. Sometimes C-Canon stuff gets G-Canon'd, like Aayla Secura.
S-Canon is Secondary Canon. This is stuff that current authors can either use or ignore when writing new continuity. Kinda stuff that doesn't fit anywhere else, or were before the time when there were efforts to keep a consistent continuity. I think the old Marvel comics fit in here as an example. S-Canon can become C-Canon if a contemporary writer uses an element of it in a new source.
N-Canon is Non-Canon. This is all the Inifinities labeled stuff and other "what-if" stories and things.
This was all created by the official Lucasfilm "Keeper of the Holocron", Leland Chee, as a system so people could actually make heads or tails of what was actually going on in the Star Wars galaxy... but, of course, all of these are obviously override-able by Lucas. If he wanted to elevate something from any other level to G-Canon, then that's his prerogative. Sometimes it's neat (Like Quinlan Vos), sometimes it sucks (for us [Special Edition]), but... it's his property, heh... If he wants to G-Canonize Melvin Fett, by God, he'll do it.
Anyway... the "Rule of Two" generally accounts for the apprentice to take his own apprentice and kill his master eventually. I don't see a problem with that apprentice deciding his own apprentice was crappy and exchanging it out for a new one - deception and treachery are the ways of the Sith after all.
The "Rule of Two" has been somewhat loose over the years anyhow, since there was a lot of canon created before the Rule of Two was laid down, so most other Sith that show up before the Rule of Two are classified as what amount to "Dark Side Acolytes" and not truly Sith.
Dooku had something of a small army of Dark Force-wielding acolytes during the Clone Wars, Asajj and Savage are just a couple of those.
Honestly, it's all really confusing, even with a system in place and I do not envy Leland his job. Poor guy has to make sense of all of this stuff thrown at him from myriad sources every day... ugh.
A fun example I like to use, because I know it so well, is how Marvel's S-Canon vision of of a jungle planet Mandalore became C-Canon when Karen Traviss used it in her quest to rape Mandalorians, and then Mandalore apparently got glassed and became a desert when Lucas made it T-Canon for The Clone Wars (there's a distant shot of it looking like Tatooine in one of the episodes, I believe), which is partially what led to Traviss having a hissy fit and storming off (yay). Though officially I believe it is classified as a "jungle planet with patches of desert wasteland where the Jedi bombed the shit out of it".
Confused yet? Yeah... fun job.