...or the "rebel"/merc units... which basically will benefit each and every faction
Mercenaries didnt always work in large groups...Yes they formed mercenary bands but a merc could always go solo...or be part of a smaller band...multiple "smaller" bands could be brought into the army and then formed into 1 unit.
Their equipment would either be replaced with army standard if their contractor wanted their looks uniformal...leave them with their own...or a combination of both [arms only or armor only provided]
Mercenary crossbowmen (for example) are 1 unit and cant be reskinned for every empire that hires them (i'm guessing they would have to take up slots for other more useful units)...that means the remaining option is to have them hired and use their own equipment ...which means when someone reskins them, he/she should be going for as less uniformal a look as possible.
(Everyone in the unit will be holding X type of weapon and wearing Y type of armor...but their looks will differ, leather armor doesnt come in just 1 design...same goes for any weapon even if its just a difference in colour [light brown crossbows vs dark brown crossbows] or design [there are millions variations in design of the dagger...but they are still daggers and not a stileto,a sword sword or something else.])
PS. The equipment doesnt have to be created by scratch if helmets, armor etc can be "borrowed" from other units with the same kind of equipment.
Imo this will provide the mercs with a bit of, much needed, "character".