What about the Thalans, they dont have arch enemies, nor do the Korx

Let's see...
The Yor chased the Iconians off their homeworld, attempted to kill every last one, and were generally unfriendly. That justifies vendetta, I'd say.

The Thalans don't have natural enemies... but they seem kinda nervous about us humans. They call a child race, and don't seem pleased with our giving away hyperdrive.

Going over to the dark side as the Terrans makes for an ironic duel with those Altarian space hippies. Humanity's good and evil sides... which will win? Evil, of course!

No, no, first they attempt to appeal to our 'humanity' in the last scenario briefing (that was a blind shot - I have never played any other Galactic Civilizations game, and apparently didn't play the game or mission where the Thalans were explained, but whatever)

Do you know what would be really hilarious? Pick 2 minor races, give them a few planets each and some high-level technology, and convince them to wage war against the other I'd love to see the Dark Yor against the Snathi - give the Dark Yor Photonic Torpedoes and Industrial Sector, give the Snathi Disruptors, Advanced Farming II, and Discovery Sphere, then stand back and watch the fun . I'll have to try that myself sometime ...

But what about the Korx