Well, I had an interesting contention a couple of weeks back that no, they couldn't, because they undergo intensive training and have a command structure, so I thought I would look up the question and see if I'd been basing my plot on a misunderstanding of the concept of a mercenary force.
Being British, when I'm looking for the absolute last word on the meaning of a word, I tend to head straight for the Oxford English Dictionary (this despite the fact that my father was a Cambridge man).
It turns out that the OED thinks that 'mercenary' means 'a professional soldier hired to serve in a foreign army'.
Well, the Cortii are certainly professional; one of the things my friend mentioned in their argument that the Cortii weren't mercenaries was the length of their initial training. They can certainly be hired, at least provided someone has the cash to meet the Cortiian Councils' expectations of payment, which, admittedly, aren't minor. The Cortii are not always, let it be noted, hired to serve in someone else's army, although that does happen not infrequently.
For a bit more clarification, I went and checked out a few more definitions. Cambridge Dictionary defines a 'mercenary' as 'a soldier who fights for any country or group that pays them'. See, I should have listened to my father and gone straight to Cambridge. That's the Cortii concept in a nutshell.
Of course, you many note that none of these definitions have touched on whether or not mercenary forces should, or should not, have a command structure, or how formal it can be before the force is automatically defined as non-mercenary. They all seem focussed on the concept of a professional in the trade of violence, available for hire.
Here's another thought to add. Current jargon is tending to prefer the term 'private military company' over 'mercenary'. Blackwater is one of the outfits commonly mentioned in this context. Private Military Companies are defined by Wikipedia as 'legally established enterprises that make a profit, by either providing services involving the potential exercise of [armed] force in a systematic way and by military means, and/or by the transfer of that potential to clients through training and other practices, such as logistics support, equipment procurement, and intelligence gathering'. Funnily enough, that seems to share a lot of criteria with 'mercenary', so I'm going to go out on a limb here and say that manually-operated excavation equipment is probably still a shovel.
So, let's go digging into the history. There's a lot of interesting stories about mercenaries across multiple continents through the ages. Admittedly the general verdict is that they're bastards, but since I can't think of an instance where mercenaries went into a fight for themselves and won, it's probably also fair to say they may not have had much of a hand in writing the histories.
Historical mercenary groups:
- Varangian Guard: As early as 911, Varangians are mentioned as fighting as mercenaries for the Byzantines, as personal bodyguards to the Byzantine Emperors. The Varangians relied on a long axe as their main weapon, although they were often also skilled swordsmen or archers.The Varangian Guard is mentioned also in Njal's Saga: "The last that was heard of him was, that he had wedded a wife there, and was captain over the Varangians, and stayed there till his death day." From Wikipedia.
- Swiss Guard: The Pontifical Swiss Guard has its origins in the 15th century. Pope Sixtus IV (1471–1484) had already made an alliance with the Swiss Confederacy and built barracks in Via Pellegrino after foreseeing the possibility of recruiting Swiss mercenaries. From 1506 until 2016 there have been 35 commanders of the Swiss Guard serving 51 popes, with interruptions during 1527–1548. From Wikipedia.
- Ninja: A ninja, or shinobi, was was a covert agent or mercenary in feudal Japan.The functions of the ninja included espionage, sabotage, infiltration, assassination and guerrilla warfare. Antecedents of the ninja may have existed as early as the 14th century, and possibly in the 12th century (Heian or early Kamakura era). The first specialized training began in the mid-15th century, when certain samurai families started to focus on covert warfare, including espionage and assassination. From Wikipedia.
Ergo, having cut my teeth (exaggeration, I got my first book on historical mercenary cults in my tweens) on mercenary histories, and done some more research today on current definitions, I'm going to have to conclude that I am in fact on pretty solid ground calling the Cortii mercenaries.
(And now I'm going to go and read some of the source material from all those luvverly articles, both because it's a best practice, and because research as a sci-fi writer is just so much damn fun...)