Thanks for the link!
That was similar to Roman Imperial rules. Only Roman citizens could serve as soldiers. However, there was the auxiliary which could be made up of non-citizens to fight for the Empire. The Auxilia "were mainly recruited from the peregrini, free provincial subjects who did not hold Roman citizenship" (see https://en.wikipedia.org/wiki/Auxilia). So if the Azlanti Star Empire was similar, it would make sense to have a "foreign legion" fighting for them (in addition to their massive Aeon Guard) that were not necessarily full-blooded Azlanti humans. Perhaps even as a way for members to obtaining their freedom or simply just obtaining "card carrying citizenship" (like a "green card" and work permit with limited, foreign rights).