Sooo here is what I’ve discovered! (feel free to edit or omit anything that doesn’t seem to work ofc! XD)

“Not much is known” apparently, about their origins which bodes VERY well for us tinkering with it to fit to our True Sith concept! *flails excitedly* (I’m also self-confessed to be TERRIBLE with dates/chronology so it may not match up with the ‘canon’ dates given in the Wookieepedia article but we can call it artistic license, right? O.o haha) 

Some history stuff (which I had to kind of link back to the actual Chiss stuff in some places in order for it to make sense haha): (working on the premise provided that Chiss were a “forgotten off-shoot colonization attempt” by early humans and the scenarios that caused (which I SLIGHTLY ran away with to tie it to our True Sith haha)

-Scientific studies suggest that modern Chiss have a genetic similarity to baseline humans, we could tweak this somehow to work into our idea of the True Sith interbreeding with ‘human’ Sith and becoming “less pure” by their standards...perhaps this ancestral species was an attempt by some of Renso’s closest/most devout followers to “reverse engineer” (via using the Force/alchemy to mess with genes and all that fun stuff) that evolution/interbreeding BACK into the original True Sith species, as they began to realise that their own species was dwindling and being out-competed by the human/Sith hybrid descendents?

-These specimens (name suggestion, something like Xiis’ia  due to the alchemy or whatever used on them in an attempt to bring them back to the perceived “perfection” of the original Sith race, still retain their connection to the Force but on a much weaker level than the remaining True Sith and the hybrids who are also changing and evolving alongside them (how else are they going to squash them back into submission later ofc! XD) are transported off the original research facility on Quensu and taken to another planet/moon, called Chufion Prime (that one near Zinuthra that we made and never got to expand on or play with? XD), to give them more space to work, setting up a network of regular transports for resources etc. Trying to go with names that make the “Ch” and “iss” sound somewhere with the idea of it eventually being morphed ‘Chinese-whispers’ style into the modern day “Chiss” species name because they couldn’t remember the full names of both as the majority of the early individuals lacked the ability to write and most of their history was passed down by ‘word-of-mouth’ stories like tribes? Too much? XD

-The project was never highly backed in the first place, and eventually even Renso grew impatient and lost his faith in it, abandoning it to instead work on the genealogies etc. that our group uncovered on their trip to Quensu, which he felt was a better way of attempting to preserve his bloodline. This left only the scientists/individuals who were MOST dedicated to the project to frantically scramble to keep it going off their own backs, since Renso and the other True Sith would eventually cut off any and all resources they had been siphoning into it (“this is a waste of time and we need those resources for ourselves! :L” <--- could begin the animosity between the Xiis’ia/their ‘caretakers’ and the True Sith (“How dare they throw us aside like garbage because they are more powerful than we are, we’ll gather strength and show them!”) The True Sith government Renso and co have established abandon what remains of that project on Chufion Prime and gradually forget that they even exist as they become so all-consumed in “we must regain control and power!” they forgot about their failed science experiments

-At the same time, the planet/moon undergoes sudden and massive freezing (think ice age), changing from a temperate climate to something similar to what Hoth is like, with the remaining specimens and scientists being forced to pack up what they could and try to out run the rapidly expanding glaciers around the planet. They aren’t AS technologically advanced as the True Sith but have access to the technology they salvage that the TS left behind upon abandoning the project. The ice eventually spreads planet-wide and they are forced to begin tunnelling underground living networks in order to escape the freezing temperatures (they tap into the natural geothermal heat underground from the earth/rocks to keep it warm enough that they don’t get frostbite and die of hypothermia? XD) This eventually leads to their skin turning a bluer tint and their eyes to pigment red, because of lack of exposure to sunlight/needing to absorb more heat slash see in the dark, you get the idea no? :P 

-They gain strength and numbers, experimenting and evolving and developing their own technology until they get to the “we can travel in space now! :D” stage where they begin to conduct exploratory forays, and come back into contact with the True Sith. By this time Renso is possibly the only (?) one left alive from the original government that REMEMBERS the Xiss’ia, but he still recognises them (kind of an “oh crap it’s those guys” moment?). He attempts to peacefully approach them but the Xiss’ia are so aggressive towards them because of their isolation being blamed on the True Sith that they attack Renso and government unprovoked. This leads to retaliation which starts the war

-War rages, and eventually ends in the True Sith beating them back into the far reaches of Wild Space back to Chufion Prime, but not before the Xiss’ia (who had to develop more advanced technology in order to survive with weak/without Force abilities, thus being more “advanced” than the TS from that prospect, but not necessarily more powerful) manage to wipe out most of what is left of the original Sith Race, leaving only some individuals from Renso’s lineage project (leading into the stuff that already happened in Act 1?), some of the Xiss’ia in the chaos flee in the other direction and end up in Known Space, eventually becoming the modern day Chiss (enter Thrawn and all that good stuff from Clone Wars etc. XD) 

-Renso leads strike teams to eliminate MOST of their technology, but the Xiss’ia leaders squirrel some away in secret and they remain on Chufion Prime licking their wounds and festering until Renso dies (Renso being their only barrier between them and Known Space, as they were too fearful of his power to try and oppose him after being beaten back several times)

-With Renso gone they become braver and venture back out to cause all kinds of problems: “GALACTIC DOMINATION MWAHAHA”?

Society structure ideas (only tiny ones compared to that brain fart up there hahaha but still a few to consider) : 
-The flee from the glaciers etc. leads to mass panic in the current population, it becomes an ‘every man (or woman whatever) for themselves’ mentality with a lot of spontaneous fighting and feuds breaking out between small ‘cliques’ of individuals

-The more dominant of the cliques overpower their rivals and establish a sort of primitive tribal government, remaining in power as long as they can continue to prove that they are the strongest individuals out there etc etc.

-This seems to work for them on the isolated planet which leads to the lifestyle becoming instinctually ingrained in their psyche (hence why Chiss are so against any non-Chiss beings telling them what to do? XD)
These ideas could somehow leak out into the ‘modern’ Sith Order during part of the war (perhaps Renso attempts to honour their “most powerful person is the leader” customs by setting up duels or tests of strength between key figures. The Xiss’ia throw tantrums when they don’t win, attack anyway and start the war mentioned above ^^?) which could lead into the development of the Sith’s original trials/’you must have more power than your rivals to be worth anything’ system?

Thoughts? Additions/Subtractions? Anything else important I should look at and add that I’ve missed?