My thoughts on the matter as a minor-league backstory expert:
A. First, I just have to say that I frankly despise the use of the D&D rules for science fiction settings or anything besides their fantasy origins. That said, a good starting point would be the little +/- in each race's description (i.e. "Strong Diplomacy/Weak Military"). A few other details of relevance:
Arceans are strong but not terribly bright.
Drengin are very bright and highly rational, but not as creative as humans.
Drath are 20 ft. shapeshifters and may be slightly overpowered (although, I have read that the shapeshifting is Altarian myth and the Drath are actually just highly sneaky).
B. All races could work together depending on circumstances, except, possibly, the Korath, who are insane by the standards of other races. Racial enemies could temporarily work together to fight a greater menace, although it would have to be a major one (Drengin and Terrans did NOT cooperate against the Dread Lords). I'd note that in your description for the plot, a Drengin character would be unlikely at best. Although I view racial alignments as applying to the race's government and not every member of that race (so good Drengin could exist), the Drengin Empire has not had a civil war in its 100,00 year history before the Korath thing, and my take on the Drengin, at least, has them as a highly loyal race, so I doubt that one would help the invasion of a Drengin planet.
As for the Iconians (possibly my favorite race backstory-wise), TOA unique tech trees help a lot with filling in their backstory. Here's the short version:
The Iconians were the only one of the younger races to develop a civilization during the height of the Precursor Era. They served and idealized the Precursors and patterned themselves after Precursor society. They created the Yor as nonsentient robotic servants. During the war between the Dread Lords and Arnor, the Dread Lords altered the Yor to have free will and a hatred of all organic life. The Yor rose up and nearly slaughtered the Iconians, of whom only a few escaped. The surviving Iconians inherited a lot of the Precursors' advanced biotechnology, but they don't really understand it, and without the Precursors' quasi-supernatural inherent "powers", they can't really control it. The Iconians have spent the millions of years since the war dreaming of their past glories, never really realizing that they weren't theirs to begin with. (As for the campaigns, the Iconians were members of the human Coalition during DL. They were the only member race to come out relatively intact after the surprise Drengin assault at the end, but have withdrawn into their space again).
C. No help here... I advise you to make some ships and take the screenshots yourself. BTW, depending on your group of players, it is worth noting that there aren't many female portraits in game, only human, Altarian, and Thalan (yes, Hithesius is female). This might be a problem, but I would note that, as these are aliens, you might not be able to tell anyway (for all we know, some of them might have more then two genders)...
D. Again, TOA techs would be very useful for this. The Drengin tech tree includes lots of "nasty stuff". Weapons can really be anything - based on the old invasion movie, both energy and projectile weapons are in use.
I'd be curious to see this document to get an idea of what you already know/have decided... this sort of thing isn't really what I think about with the backstory.
BTW, reading some of the more "fanfictiony" AARS might help with inspiration.
