The United Empire isn't at all xenophobic or human-centric. They want to expand and assimilate as many different peoples and cultures as possible (i.e. meet new peoples and turn them into new patriots). So, it follows that the Yuusho with the UE on Raia are considered full citizens.

Fair point on the xenophobic/human-centric remarks. I'll edit the post accordingly.

What I'm really looking for is HOW the Yuusho became part of the United Empire? Have they been on Raia longer than the Mezari have? Did the Yuusho just show up one day? And how did they participate in the Great Continental War that created the United Empire?

While I don't think we have a specific explanation as to how, I have a feeling that if they are so far from home on Raia when the UE isn't spanning multiple planets that the Endless were involved, so most likely either the Mezari brought back some on some expedition, or the Endless kidnapped some and the Mezari got their hands onto them, perhaps Raia was even one of the locations that the Endless experimented to create the Hissho on, and some Yuusho were left over. From the way they are integrated into the UE, I'd imagine that them being there has had tremendous impacts on the Empire's politics and general societal views, as they make up literally a third of their starting population. Give the general expansionistic ideology that reminds me of the Roman Empire, I'd say they might have been there from the roots of the empires history, and may have even helped originally found it given how some of the Hissho's and UE's beliefs are very similar.