Colonisation and the CortiiGiven the generally cold and occasionally fissionable-hot relationship between most of the humanoid governments and the Cortii, it may not be immediately obvious that there were Cortiian units on a lot of the early colony ships. And then, if you think about it a bit more...humanoid governments had been hiring Cortii to do their dirty work since long before the colonisation waves, and dealing with new things is inherently risky. Having some heavily-armed, survival-trained, and cynically-minded mercenaries aboard to drop out of the airlock first can pre-empt so many problems.
'A testing environment solves many problems.'~Training of a Cortiian
Initially, there were the interstellar drives - sub-lightspeed, because lightspeed, increase in mass to infinity, etc., etc. From whichever of the Central Worlds was the original homeworld (no one really wants to solve that argument), exploration ships took the long trip at somewhere between half and two-thirds of light-speed to other rocky planets in the original solar system, and set up bases, experimented with air scrubbing, water recycling, and food production until they got good at it, and finally took the sideways step into terraforming - with more and less successful results.
From there, with a lot of the basic experimentation done, colony ships were sent to nearby solar systems. Since absolutely no one really wants to settle once and for all which of the four Central Worlds was 'the' Central World, stick a finger in the hologram on whichever you like. Those ships also had Cortiians aboard.
At some point after that, researchers stopped banging their heads on trying to solve infinite mass versus propulsion, and had a breakthrough that resulted in point-to-point travel, or as it's more commonly known, deepspace drive.
This resulted in the First Colonisation Fleet, which would fall firmly into the 'unsuccessful' category of colonisation attempts.
'In the hands of a fool are all things foolish.'~Sayings of the Wise
The First Colonisation Fleet
Given primitive humanoids and their tendency to breed indiscriminately, it shouldn't be a surprise to hear that a lot of the incentive behind the development of the original deepspace drive was to solve a massive overpopulation problem. Population-wide contraception actually preceded it by a few generations, but by that point all the Central Worlds were pretty much teetering on the point of not being able to support their populations.
With the advent of the deepspace drive came another massive incentive: hail conquering heroes, go forth and be granted as much surface space as you can possibly manage. The governments of the time didn't need to resort to deportations - they had more volunteers than they could build hulls and suspension tanks for. Private initiatives sprang up across Central space, building deepspace ships and offering space aboard.
Records of the time, given the sheer numbers of parties involved, are contradictory, but somewhere between six hundred and thirteen hundred experimental ships vanished into deepspace over a period of a hundred years, each carrying several hundred to several thousand aboard.
Even some of these had Cortii aboard, due largely to hazard bonuses and pre-payment contracts. Even the healthiest culture of cynicism is soluble in enough credit.
However, given experimental drives and the fact that the numbers of ships leaving Central Space in every direction vastly exceeded the number of planets about which long-distance research and exploratory probes had more to say then 'we're pretty sure there is something there', only a fraction of that First Colonial Fleet actually resulted in stable, high-tech colonies.
Miners and scout ships in remote locations still occasionally trip over drifting wrecks, and first contact teams have discovered several humanoid populations on outer-system planets with some interesting gaps in their fossil records, a really big impact crater, or stories of ships that carried wisdom from a distant land.
'Coincidence is the crutch of optimism.'~Training of a Cortiian
The Second Colonial Expansion
...might fall into the semi-successful category. Much better controlled, with destinations that at least rated a definite maybe on being terraformable, or stable enough to support a station habitat, twenty systems were selected for the initial wave, reconnoitred on a detail level, and finally approved for colonisation.
Not to mention, the deepspace drive had had a couple more centuries of fine-tuning. All twenty ships made it, one got blown away by defences the probes had missed, two turned out to be station prospects rather than terraforming prospects, but overall it worked. Most of those twenty ships carried one or more Cortii aboard.
In the interest of accuracy, it should be noted that the defences the probes had missed were in fact Base Zero; the Cortii had a sizeable fleet of their own and substantially less bureaucracy. The Central Worlds government declined to believe that there was a Cortiian base already in the system, but their ship went in heavily armed nonetheless. In the event, not nearly heavily enough.
Those colonies, in turn, spread, and split over time into the various political factions that form the basis of current Cortiian employment - pardon me, today's civilisation.