The best way to get an understanding of the Sith before the Rule of Two is to read the first Darth Bane book. It's a quick read and is the basis for the Ro2 and why Bane started it.

Essentially though it only really differs in that it dispenses with an open Sith hierarchy and the actually running of a separate Empire, think feudal Europe with Darths and Lords being Kings, Princes, Dukes etc. and such. Most of the Sith were sort of like Anti-Paladins vying for position in the hopes of reaching Lord or Darth status, mostly by killing each other off. The Sith Empire itself was a corporate oligarchy where the citizens were ruled by non-Force using administrators.

As for Lightsabre schools and the other stuff, I'm not sure it's actually spelled out but I'd assume they were similar to the Jedi, there are only so many ways to swing a blade after all. They did generally use synthetic crystals which I'm sure would have a variety of compositions that would mimic natural ones.

The Sith Sorcery and Alchemy, well that's going to be complicated for because there isn't anything currently in the RAW to base it on.

The thing that separates Sith from Dark Jedi is that the Sith have a philosophy in the first place. They have this whole thing about personal strength, power through ruthlessness and so on; Dark Jedi are merely former Jedi who have turned to the Dark side. No particular mindset or philosophy required for those, since they're typically individuals who have gone bad.

This may help: the Sith Code, written by a Jedi heretic and founder of the Sith Order proper:

"Peace is a lie, there is only passion.
Through passion, I gain strength.
Through strength, I gain power.
Through power, I gain victory.
Through victory, my chains are broken.
The Force shall free me."

It's interestingly set up in direct opposition to the Jedi Code (interestingly the Jedi Code at the time was the much more balanced "yet" version, as opposed to the more strict "there is no..." -a huge part of our campaign is our group of Jedi Younglings trying to reform the Order pushing it back to the "yet" version of the Code and are sort of somehow ending up going back to the more balanced view of the Je'daii Order)

The thing that separates Sith from Dark Jedi is that the Sith have a philosophy in the first place. They have this whole thing about personal strength, power through ruthlessness and so on; Dark Jedi are merely former Jedi who have turned to the Dark side. No particular mindset or philosophy required for those, since they're typically individuals who have gone bad.

True, but some groups of Dark Jedi have unifying philosophies, and for some (like Dooku's Dark Acolytes), that philosophy is based upon Sith teachings.

The thing that separates Sith from Dark Jedi is that the Sith have a philosophy in the first place. They have this whole thing about personal strength, power through ruthlessness and so on; Dark Jedi are merely former Jedi who have turned to the Dark side. No particular mindset or philosophy required for those, since they're typically individuals who have gone bad.

True, but some groups of Dark Jedi have unifying philosophies, and for some (like Dooku's Dark Acolytes), that philosophy is based upon Sith teachings.

Absolutely. I just meant that all Sith more or less subscribe to the same philosophy, while Dark Jedi may have wildly diverging philosophies (if they have one at all beyond "more power").

Is the Sith culture even well explained in the sources both Legends and the new canon can provide?

Here is the best way to play a sith.... Be a **** and be careful of those that are stronger than you.

Essentially be that guy who pushes a kid down cause you want him to try and kill you in the future. While you are at it murder his family for a glass of blue milk.

• Carry a red lightsaber at all times.

Is the Sith culture even well explained in the sources both Legends and the new canon can provide?

There are actually a number of good sources for the ancient Sith, most from the old EU, i n particular the Dark Horse Tales of the Jedi comics, which deal with several key wars between the Jedi and Sith from 5000 ABY-about 3960 ABY. Golden Age of the Sith, for instance actually focuses on the ancient Sith Empire from 5000 years ABY before their fist war with the Republic.

Might I further inquire whether they shed light on the difference between dark Jedi and the Sith? What I have stumbled upon thus far suggests that the only difference between them is that pre-Bane they belonged to a large organisation that called itself such, and post-Bane you are a Sith if you are called such by the ones who were Sith before you.

There's a lot more to it than that, and to really grasp it, you need to read the comics, but here is some info you might want to read. It's a good start to get at least a basic grasp on the Sith, and their beliefs.

The Book of Sith is now Legends, but I would urge you to read it. It's made up of a variety of excepts of in-universe books written by historical Sith from the time the first Dark Jedi founded the Sith Order to the fall of the Empire.

Sith sorcery & alchemy aren't currently covered by the rules, as FuriousGreg stated. You could try to adapt material from an earlier RPG or just hand-wave it, depending on what the players are trying to accomplish. The problem you run into is balance... a character in a book, particularly a Sith, isn't worried about game balance, merely the accumulation of lore & power. In an RPG, though, you need to try to keep power levels equivalent between classes, provide costs and/or drawbacks to ensure that there are no "auto-win buttons," and generally dig into all of the behind-the-curtain mechanics.

Good luck.