As far as toxic oxidation and oversaturation is concerned you would have to look at how a species evolved within its environment to answer that one. On alien worlds things are not going always be like here on Earth anyway so the sky's the limit for these things.
There are animals here on this planet that have evolved over millions of years as the planet went through its various stages of development atmosphere-wise among others. If you went back to the Cambrian Era and brought forward a critter from there it would likely die from a lack of oxygen. Likewise if you went back there you'd die from the higher content of oxygen burning out your lungs.
All those things are relative to so many factors that this why you can range all up and down the "hard sci-fi scale" and get away with it most of the time. In a book you might not, but that depends on the target audience and you can be surprised how much weight some lamps will support anyway.
As far as your Traveller game goes: do thou what thou wilt.
Mine is entirely non-OTU (except for the rules) and has an evil (as far as humans are concerned anyway) repitle-like mammalian race that is constantly fiddling with its own genetics to create a race that can better compete with its opposite: the Terran Empire and those darned versatile humans. It is a caste society and clones human psi-talents to use as spies among the humans.
A race of felines that were artificially uplifted by one supertech race a long time ago in order to use them as soldiers to fight against their supertech rival...and then when millennia ago both of those races moved elsewhere these cats came up with their own myths and culture and continued advancing (though only at TL-11 so far). The best part is that they don't know the real story of their origins, but they have heretics who come along once in a great while who stumble across it while researching certain inconsistencies in the species' makeup. They look more like a cross between a lynx and a cheetah, and are much smaller than an Aslan (which I abhor more than those talking dogs and starfish), but the clans/families have been selectively breeding for bloodline-specific psioinc talents for generations. While they are imminently capable of fighting to defend themselves, they have developed a peaceful society that emphasizes trade and inter-family cooperation. They construct vast "scent-story" gardens (some on the homeworld are the size of cities and tell th onstantly advancing story of thier race), the warriors wheel about in huge manadala-like formations to the sound of kettle drums and chants to practice coordinated maneuvers in a tradition of individual close combat over mass tactics, and the race uses different drive tech than everyone else. Currently that's stutterwarp.
A race of hyena-like marsupials who were nocturnal omnivores that evolved to have semi-intelligent females, a certain racial inferiority complex and strong desire to spread out through all of known space by means peaceful or violent. Due to the violent approach not working too well most of the time (except when picking on the colonists settling among the worlds formerly contained within the now-defunct Terran Confederation) they are changing tactics to use cunning and guile. They also have some of the best mercenary legions for rent, but it's hard to get them to fight each other. They prefer not to but relish fighting human mercs; it has a lot to do with the racial inferiority thing.
Plus a myriad minor races in a universe I built from scratch in '77 and have been tinkering with, players or no, ever since. It's worked without a lot of whys and hows from players for all this time, with very few exceptions, so I think if you come up with Blue Darrians and fit them in what I consider a goofy OTU I think they'll work fine without you having to sweat all the minutia.
I do agree the Traveller OTU is a bit goofy, it is what it is though. lol
I like the minutiae, for example i am working right now on an art piece showing how the Blue Darrians originally built tanks out of common mining vehicles. I do it for me mostly, but if other people like it, I'm happy to share.
Personally I find the OTU too nice, all the races and the 3I is too foo-foo. IMTU after the 5FW, it is shown that Strephon is a weak Emperor, and a new group arrises amongst the nobility and naval high command called "the Young Gods" from a psuedo secret society they themselves call "Guardians of the Imperium". It much more gritty and realistic setting, with the 3I being more militaristic and aggressive, while propagandizing themselves as Free Humaniti, and espousing a philosophy that they are the only legitimate rulers of the Human Race.
After the 5FW the empire realizes it can't just let itself continue to be attacked, so the GotI come up with a plan of the "Grand Fleet" using what they call the Jump Fleet Strategy. They start a war with the Zhodani, The Zhodani Punitive Expedition, letting the Zhodani build forces and incur on the Spinward Marches, at which point a massive fleet jumps in and breaks the Zhodani fleet. This follows on with a massive attack on the Zhodani Consulate, tearing a huge chunk out which the restyle the Zhodani Republic, a client of and occupied by Imperial forces. Major planetary defenses are taken out in "Carrier Raids" through Zhodani space "Zod's" in military slang. This forces a peace treaty on the Zhodani as the the Grand Fleet seems to menace the Zhodani core worlds with deep strike attacking their industrial base. Then the Grand Fleet turns 180 degrees and attacks the Solomani Confederation in what they call the war of Re-unification, after they have done political intrigue to weaken the Solomani.
This leaves mop-up and counter insurgency work in the Zhodani Republic to the local forces of the Spinward Marches. The Thought Police being called "Mind Terrorists" and nobility "Slavers" and generally rounded up and executed. The local Zods fight it out though. In one story I'm writing the Zod insurgents hide a Grav Tank in the sewer of a town and when a patrol sweeps in the Zods blow the road and the Grav Tank hovers up out of the hole and only by the action of the Platoon Sergeant spinning a plate mine out of a "Contra-Grav Explosive" (like TDX but the explosive force travels opposite to the gravitational pull) knocks it out, losing his arm in the process. The arm can and will be regrown at a rejuve center, he questions going back, the war seems endless, but he does believe in the ideals of the 3I. He goes back for the brotherhood of his unit eventually, they need him and it is his place in life.
All of this disturbs the other races in the known galaxy. What will a new and aggressively expansionist "Empire of Humaniti" mean for them? They don't want to operate directly against the Imperium for fear of it's new strength, nor do they want to caste themselves in the light of being "anti-human", if and when the Imperium wins these wars. This opens up a huge number of new worlds for intrigue and adventure. Players can be either Carpetbaggers or Soldiers in the Zhodani Republic, sometimes against their will as the Imperium will take people convicted of crimes and commute their sentences to service, because the 3I needs warm bodies to fill out slots in administration and military units.
It's gritty, there really aren't any goodguys, nor manichean absolutes of good and evil, much like the real world.
