If the mercenaries absolutely have to meet an Ethereal face to face, then they wear some form of suit at all times, to prevent any influence to cloud their judgement. Synonymous with the Mont'au, the terror of Tau history before the Ethereals enlightened them, the Ruinous Powers have never truly been a great threat to the Tau race. There have even been instances where the mercenaries tried to convince the Empirical Tau to abandon their Ethereal masters and earn their freedom by fighting for the Ar'Cyr'Ru Fleet. level 1 Even elements of the Ordo Xenos Inquisition saw use in hiring these Tau. There were times when Tau worked alongside the Dark Eldar in times of direst need, but these Tau know well enough that trying to establish relations with the fallen kin of the Eldar race will bring nothing but dark omens upon them. Hive Fleet Gorgon. Ever since they were branded as renegades, the Ar'Cyr'Ru Fleet had rarely entered the space of the Tau Empire. Their doom would have been sealed if it weren't for Shas'O Dal'yth Su'Nan'Ol. Most notably, the Ar'Cyr'Ru Flotillas were known to cooperate with Eldar Corsairs as they sail the stars in search of glory and accolades.
This individual, known as Shas'O Vior'la Cal'Elan, made these Tau realize the importance of infantry on the battlefield and the fact that close-quarters-combat is unavoidable. Desecrated Stronghold. The full list of all Tau vessels is here (the numbers are only roughly estimated, the exact number is unknown): The standard Demiurg rarely come to fight on foot, preferring to use their battleships to hammer their enemies from space or low orbit, but there are times when these Xenos take up arms and deploy directly into the midst of battle.
This Council was comprised of four individuals, each a representative of their Caste within the Ar'Cyr'Ru Fleet. These "war trophies" can come in the form of stolen gear, such as an Imperial Auspex, or trinkets such as Eldar Spirit Stones. Occasionally the Flotillas might uncover a call for aid and they will respond with all haste in order to assist their brethren.
The art of double-crossing is something the Ar'Cyr'Ru Tau are very well accustomed to. There have been reports of Demiurg Bastion and Stronghold Class Commerce Vessels following the Ar'Cyr'Ru Flotillas, acting as trade partners and even helping the Tau warriors when a contract is signed (with a share of the payment promised to them). It is not common for Vespid to travel beyond the territories of the Tau Empire, but in rare situations when they do they'll not fair well alone. A bare few were even slaves, which the Tau accepted as payment, but were set free from their chains and allowed to work with their saviors. The Tau Mercenaries are often accompanied by a number of other Xeno races.
Sadly, like its Swordfish counterpart, the Malo Gunship is a rarity in the entire Ar'Cyr'Ru Fleet. After the fighting is done, the Tau will request for adequate payment. Although no Ar'Cyr'Ru Tau had ever even met Puretide in person, they all deeply admire him and respect his prowess as a military commander.
Although the Ar'Cyr'Ru Fleet is divided across the Ultima Segmentum (and is even found in some areas of Segmentum Tempestus), they do have several "Harbors" as they refer to them as. This paranoia goes as far as the fleet members refusing to negotiate with an Ethereal in person, but instead communicate through technological means, like a monitor or comm-links.
These Xenos survive through trade, selling their alien technologies for supplies necessary to continue their voyage through space. These relations can also extend in battle, where the Demiurg will spare some of their miners to fight with the Tau in whatever war they are waging.