The biggest issue I have with unique mercenary units is being able to limit them. They will only be possible if that can be done.... otherwise you could end up with multiple versions of a unique unit.

Where mercenaries are more 'generic' that is not an issue. No reason why we can't have a few 'extra' units without proper factions thrown in as mercenaries. Lots of scope for things here, and as long as the rules don't say a unit could not be a mercenary unit, then there we are!

Ultimately, there will be a limit.... and I can't see there being a huge number of variations ... but there will be some! The same basic units will also serve as rebels where no suitable units can be adapted and reskinned