We CAN actually buy mercenaries in GalCiv2!
thu a clever twist in existing gameplay...
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GalCiv2 Forums
Simple: Buy another civ's troop transport!
Its highly likely that some people has already discovered this tactic, but probably it didn't hit em yet that THAT is in fact shopping for mercenaries... Yes, we are buying the ship, but we also get the soldiers inside it for our own use... what more, if we buy one that is parked in orbit, the ship will come stocked full with soldiers taken from that planet!
Can use em to conquer others, or as a quick way to grab populations from other civ to bolster our own... I used this strategy in my last game, where I buy troop transports by the truckloads and transfer em to my newly conquered Arcean homeworld which has tons of economic buildings and Econ Cap... in that single turn, i invaded the planet, and populated it to 10 bil pop and it starts rolling in wads of BC right away...

as to how many troops you get, if it is launched you get what it has onboard at the time. if on planet it gets up to half the planet pop, but obviously not more than it can carry. i use it to a) stock up for wars ,
seems they get upset by the parade of mercs passing near their territory. growing your planets with mercs can cause morale problems if you are not careful. those mercs in their territory also degrade your ability to trade with that culture as long as they are close enough to be a threat (diplomatic trade, not merchant trade).