Well, there are no other human mercenaries. The Grathik went and grabbed elite humans from the getgo because they needed a force multiplier, and they didn't want to grab divisions and divisions of human troops. Since the contractor community tends to keep to itself, they decided to pick exceptional specimens out of that community on the idea that saying they went to Uzbekistan (a country known for clamping down on media reports and the such) would help cover the activity. Maybe not completely, but they didn't want to spook humanity with disappearances.

Also, start small, make the first wave of elite mercs very rich and Humans will be signing up for a tour afterward, assuming they come back.