Which, if any, OTU races do you use or would you use IYTU?
I use both the Hiver and the Droyne as the primary alien races in MTU.
Culturally, they are pretty much as given in Trav. Historically, they represent the two large, well-established, millenia-old races that dominate this galactic neighborhood.
Terrans have long since learned not to trust either of them. The Hivers and Droyne are locked in a very long-running cold war, and they each have their respective designs on the Children of Earth. Hat tip: Babylon 5.
Hivers were the Terrans' First Contact, and have been lying about everything since Day One. The Droyne, on the other hand, while more consistent in their dealings, are aloof and inscrutable and give Terrans the heebie-jeebies because of the incessant low-level, paralyzing-fear-inducing, predatory psionic emanations they rather discourteously give off second-naturedly.
Plus, both species present as truly "alien" -- whether it is the eerie hush of the mute, tentacles-everywhere-and-into-everything Hivers, or the always-watching bug eyes of the occasionally clicking and twitching raptor/bat/mantid Droyne; they are both clearly
not just humans in rubber suits.
And speaking of Humans, note I refer to "Terrans" in general. I use the Vargr, the Aslan, the Ursa, and the Tursiops -- again, culturally as given in Trav -- as Uplifted Terran Races (mostly to offset Human guilt at extincting all the other primates before First Contact, but the Vargr and the Aslan helped with the later Uplifts, too). Hat tip: David Brin.
Along with the Humans, the Vargr and the Aslan are "Major" Races, while the Ursa and Tursiops are more "Client" Races without much starfaring capacity (or interest in it) of their own. The K'kree are likewise a pacified Client Race of the Hivers.
The three senior Terran races (monkeys, puppies, and kitties) have established a mutual non-aggression arrangement, and have headed out from Terra in mostly-separate directions to make their own ways and build their own interstellar civilizations. There is barely even any family rivalry between the Terran Races -- plenty of internecine conflict
within each Race, but a good faith effort to avoid interspecies conflict and maintain more of a friendly rivalry (and mutual defense pact, if necessary).