Two scenarios come to mind. A) Ancient magic is heavily regulated and punished within elven society. Among the lawbreakers, amicable ones are chosen to become Canaries. The problem is how to come up with enough amicables, if real-world trends apply. A huge prison population to pick from, all guilty of using ancient magic? Bonds of friendship forged in a gnarly training regimen (does Pattdohl really seem like she could handle one?)? Mind control? Some unavoidable punishment for any that go AWOL? It's strange that convicts in service have such free rein. Maybe license to mess around with other races' affairs is more interesting to them than rebellion? It's hard for me to see this working.
or B) Though there are laws against using it, knowledge of ancient magic is ubiquitous among elves. Breaking magic law is akin to a slightly disreputable rite of passage, like running away from home to join a circus. Except in this case, your career choice is turning yourself into a weapon at the cost of becoming a state secret. I think volunteerism better explains how friendly everyone in the squad is with each other.