so you want to frame it as a post-production "behind the scenes" special about the fictional TV series of X-Com? fascinating.
if i may offer some input... as to the extra alien races: just remember that even babylon 5 introduced a new alien costume about once per season, (Drazi, pak'ma'ra, Drakh, and the Gaim) likely due to the budgetary restraints of having to make 20-30 "similar but distinct" costumes.
hell even SG-1 used humans without costumes en masse (the Goa'uld, Tok'ra, Jaffa, the replicators, hell, even the ancients themselves) and on the rare occasion you DID see a truly different alien it was either CGI or it was just the one guy in an Unas or Kull costume (tho eventually they made like 20 of each of those, too)
The vulcans/romulans were just humans with bowl haircuts and pointy ears, and the klingons were just large humans with KISS outfits and paper mache on their foreheads - for a reason: The less human the costume, the greater the cost. heck even the borg looked like they were only there to soak up the overflow from the props department.
essentially what i'm getting at is: from a space opera production perspective, each race has a significant budgetary requirement attached, mainly due to needing at least 20 of them to stage a convincing firefight.
Considering that space operas are almost universally under-funded, and generally spend most of what they DO have on writers and CGI, the racial diversity tends to be fairly limited until the third season.
