Why did this come up when I'm away from my computer? Lol
Much of what I laid out in my IMTU mods to the Wiki description of Collace (
Collace Between The Lines) ought to still apply (you can skip the ongoing terraforming project started by the Darrians, but I think it adds a nice bit of flavor here).
The main things I'd keep from MTU are Trexalon's environmental issue
not being that it's air has frozen out into oxygen and nitrogen ices. Instead, while it really does have an Atm-6 (canonical), it has almost no water (Hyd-1, canonical) and orbits a gas giant (canonical)
inside the gas giant's radiation belts. Without an artificial geomagnetic field (in the form of a charged plasma ring around the planet), the radiation would be lethal to pretty much anything higher than monocellular life. The Trexalon Technical Consortium (TTC) (canon) uses control of the water supply and the geomagnetic field maintenance to sustain its hegemony.
That and, much of Trexalon's population (IMTU but it seems the only reasonable explanation) originated on Pavabid. Originally, these were overpopulation-driven migrants from Collace to what would become Pavabid, then forced to flee Pavabid in the wake of its theocratic revolution. Collace would not take them back, but did help transport them as well as establishing additional arcologies on Trexalon for them (which the TTC promptly seized). There's great resentment about this among the descendants of these expatriates.
As I mentioned in my preceding post, Trexalon and Kuai Qing are going to be their own thing, and it may not be worth forcing the issue. Trexies will be looking to expand their influence into Egryn and Pax Rulin subsectors (with Motmos and its Naval Base being a significant obstacle to this objective). Their (J-3) trade routes to Rimward/Trailing go through Kaldamar (TR1201) and then Carben (TR 1502) as a "gas-giant refueling stop only" due to the red travel zone. TL-12 Trexalon can build (indigenous naval production, per LBB5) J-3 to reasonably high tonnage, but their cost-efficiency is limited by having only TL9-12 fusion tech. Unfortunately (as noted above), I don't know how Mongoose's "Sindal campaign's" canon affects that potential expansion. Haven't read it, but it appears they developed it coreward from Sindal subsector so perhaps there isn't a lot of overlap yet. Maybe there is, though... [Edit: and apparently they're rolling the 5th Frontier War through here as a bridge between Sindalian Pirates and Fifth Frontier Warriors and/or Refugees, which means they're likely to solidify their take on canon throughout the subsector.]