Have we ever seen actual onscreen evidence for soldiers for hire in Star Trek? The closest one I can think of was in the first season of DS9 when the guy tried to steal the Dax symbiot. He had at least one Klingon hired to help him. And although the novels are not canon, the TNG novel "Survivors" had the former mentor/lover of Lt. Yar having become a mercenary after he was courtmartialed and thrown out of Starfleet (by mistake it turned out). It seems to me that given what we know about the Trek universe it would be literally awash in soldiers/starship crewman for hire. Klingons who get tired of "chasing honor". Trained Starfleet personnel who can't abide by Federation rules and regulations. Demobilized former soldiers from say planets like Eminar VII or Ekos who don't have anything to do once peace breaks out.