Mercenary companies are used throughout the Galactic Union, and are regulated by the Mercenary Guild.
Of the various races within the Galactic Union, only 37 have the requisite physical and mental capabilities to serve as mercenaries. Although there are occasionally a few members of a non-merc species who are willing to make a living as a merc, they are usually considered abnormal among their own races.
Most mercenary companies include only one race, because most companies are controlled and regulated by planetary governments. However, some companies include a handful of aliens in their ranks.