Okay... I may as well put in my two cents... The following is as close to what would happen given common sense and the social structures involved.
The Unity (didn't call themselves the advent until later) was on a remote desert planet. When this colony was about to join the TO, ambassadors and diplomats went there to fill in the legal blanks. What they found was an entire race of people who had used implants, genetic manipulation, mental trawling, among other taboo techniques to make themselves better. Self improvement is a good thing, but according to the TO, illegal. More than likely, the TO debated for a long while what to do with these people. Some wanted their planet leveled, while others didn't see any laws broken, so they said to leave them there. Eventually, it was decided to exile them into space. The Unity did not hear the reports of those who wanted them to stay, only of those who wanted them dead.
Without a planet, 2-3 billion people (about the max pop of a desert planet) would have great difficulty surviving. Over the next few centuries, they suffered from the possibility of starvation. Eventually, a system with terran planets was found and colonized. At this point, the Unity began to expand. Without traditional moral guidelines to hold them back, they began designing new and more powerful weapons. Soon, they began to experiment with the crews of TEC scouts as they came into the Unity Space.
As far as the TO was concerned, these ships were encountering some sort of a threat, so travel to that sector was barred. Without more subjects, the Unity was forced to ensure that they had a steady supply of test subjects, so they began to use yet more taboo methods. They let these subjects "live" on a planet until one day the Revelation Battlecruiser was scheduled for testing. Researchers believed that the powerful PsiTech of the crew would be enough to carry influence over entire planets. They were right. When the ship's collective mind tried, it was able to provoke hysteria. They were soon forced to stop, else all their subjects would be destroyed, ending their fun.
Eventually, they decided to leave their subjects alone for a century before coming back. When they returned to "check up" on their lab-rats, they found a "fleet" ready for them. Unfortunately, the rebels were crushed quickly by the new experimental axial gun of the Radiance Battleship.
There was one odd occurrence in the battle though... There was a crew of members more than two hundred years old. All of them had mastered their PsiTech, but the rebels had managed to focus all their fire on the single Halcyon Carrier they were within. In the brief moments before the death of the ship, the crew of a Progenitor Mothership managed to transfer the collective consciousness of the Halcyon to another, inexperienced one. Even with the death of their original bodies, the master crew was able to survive.
----- Two Hundred Years Later -----
After developing countless weapons that would surely spell disaster for the TO, the Unity, or Advent as they would come to be called, launched their entire force towards the central worlds. Upon arrival, they found their target, the TO, to have become the TEC and to be fighting a war elsewhere.
Without any ships to defend against it, many central worlds were exterminated. Even the capital, Naeve, was destroyed. The Advent had done more than their predecessors had received, but a thousand years of dwelling on their hatred caused them to believe that the death of a dozen planets and 50 billion was not enough. These sinners had tortured an entire race. Now, they would pay for it with their lives.
And that is how I see it happening...
Oh, and for the record, it does not say that the Unity excludes men from their equivalent of politics, they only say that women are more common. Now, for a race that is dominated by PsiTech, this would mean that men would traditionally be looked down upon. However, there would be some who are determined enough to train themselves and rise through the ranks and more than likely, the Coalescence, the Council, whatever you want to call their governing body, is run by men. So, the rankings would likely be:
Coalescence: man
Council: men/women (mostly women)
Capital Ship Crews: women
Cruiser Crews: women with the occasional man
Frigate Crews: women with a handful of men on board
Scout Crews: about 1/3 men
Physical Laborers: men (for obvious reasons)
The unity is all about efficiency, and women (as psychologists and employers already know) are more efficient than their male counter-parts. Women tend to work more and goof off less. For the Unity, this would only be magnified. In history, races run by women (what few there are) tend to be more advanced than those around them and also peaceful (although they tend to get wiped out due to the lack of an army). For this reason, the Unity was peaceful until their expulsion. Any guy with an ex-girlfriend can tell you that it is not wise to get on a girl's/woman's bad side. Guys get mad and then forgive each other the next day. Girls tend to hold grudges... For a long long time... Once again, for the Unity, this would be amplified as all members carry the memories of the previous generations. For this reason, when the Advent return, they are ruthless. They still feel the pain of the expulsion as if it was yesterday.