When you hire a merc, part of the cost is that you are paying the mercs to let other mercs in your service know their strengths and weaknesses.
Mercs should not be cheap. For a bounce infantry regiment (Battlesuits and grav belts), you can figure at least Cr1000 per day per troop. The suits need replacing every few years, plus you have to feed and pay the troops. Food is about Cr20 per man-day, but add the "2000 days of combat duty" life of a battlesuit, at MCr1.5 each suit... that's Cr750 per day amortizing the cost...
Pay is likewise Cr50-100 per day.
You lay out huge expenses to form the unit. Your employer pays you for being ready to go and being already trained and equipped.
In one game I ran, I had a merc unit of Bounce Infantry being played (PC's as cadre of the unit; senior staff). They charged KCr3 per day per troop. Plus transport, Food, and travel pay. This worked out to be (for a 1/3 staffed regiment) over MCr2 per field day... and they charged way more for drop ops.
Grunt infantry can be had far cheaper, if the environment allows.
You field units like this at the company, battalion, or regimental levels; these units are internally cohesive, and usually long-standing. You put units together under NON-MERC superiors, and then tell the mercs what you want done. The superior DISCUSSES with the mercs who and how, and makes a decision. (Or, if stupid, or arrogant, the supervising officer orders the unt he likes least to do the task...)
A Merc unit that takes significant losses would almost never be "Combined" with reinforcements from other units... Relieved by other units, yes.
IMTU, Merc Units need charters from the SS or Sector Dukes, or in rare cases, the Archduke. This is a charter system. The charter is a two way agreement: The holder gets to have military hardware, and may engage in contract as a unit of mercenaries, for actions within the area or against threats to the imperium. It also requires the unit be trained to the minimum standard, and that, in timme offf war, the unit will be mobilized as part of the Imperial Army.
Becuase of charters, most mercs have long histories... and unit loyalty is seen as a point fo pride.
YMMV.