Even a thief has a certain set of rules that he will aim to never break, simply out of respect for his own job and himself as a sentient being. I don't expect any less from a mercenary.
It is but an obvious outcome of unjust actions, that mercenary who decides to betray his employer, will have his reputation severely damaged. I don't suppose I have to explain what it means for a mercenary to have a negative reputation bound to his name? Suffice to say, he should start looking for another job.
This isn't to say a mercenary is going to have chances in tier-advancing proportional to those of other members. No, he will not find himself occupying higher (namely leadership) positions. But in truth, what would be the point of the other state of affairs? Mercenaries aren't full-fledged imperialists, while their function more often than not, isn't going to require an official command over other members. The hierarchy of the Dark Empire is meant to provide functional solutions in running the guilds that remain under its command. What follows, the purpose of our hierarchy isn't focused around making it possible for the members to get promoted in no time. There's much more depth in there. Occupying higher tiers burdens with much more responsibility and for this reason standing as one of the officers has both its advantages and disadvantages. We, as the Dark Empire, realise this fact, and therefore it is our job to explain the same to the aspirants, preferably by showing them how the Empire works.
Now, if someone wishes to advance, and at the same time shows quite a bit of skill in leadership, rather than remaining a mercenary, he should look into advancing as an imperialist, which may be achieved by changing the direction of his function/rank development. I see no problem in there.
As to the payment itself, as has been already explained, we guarantee training and sustenance. What the training will look like, that's partially explained under the Way of Steel link. For mercenaries, standing successful in what they do isn't going to remain unrewarded. The reward however will be rather based on our current economy and possibilities provided by Planeshift itself.