Some good points came up in the comments about last night’s quick thought. I thought I’d carry on here though as I wanted to make some additions.
Firstly, while we may do a whole mercs army at some stage, for the moment we need to focus on the main factions. That means that mercs aren’t the main event here, they’re the cherry on top.
In a background sense, especially in campaign terms, I don’t see any military unit progressing and being well regarded if their work is all done for them by hired hands. For that reason I’m tending towards being quite restrictive with the amount of help they can give. One brutally simple option would be to say that forces which included mercs cannot advance as a result of that mission. Individuals may skill up, of course, but the force as a whole wouldn’t as they weren’t actually doing the work themselves. Maybe that’s too much though.
Another way would be to say that mercs cannot earn VPs for any mission goal. This is what I mentioned yesterday. The logic holds for any goal as having your work done for you doesn’t differentiate about the work. And would you believe them anyway? “Yes, of course I killed, oh, 5 or 6 Stage 1s. They’re just around that corner. You can’t see them because the building collapsed on their bodies. Can I have my bonus now?”. Yeah, right.
Even if you did believe them, whilst achieving the mission may be great however it is done (from the point of view of the overall command), it isn’t going to earn Brownie points for the unit involved if they rely on hired guns. It might make the merc more famous though. I’m sure they’d love that. However, we’re not playing a game about a single merc (that would be an entirely different game), we’re playing a game about a strike force from one of the main factions. As such you have to look at it differently.
Individual units don’t have the cash to hire mercs. They’re paid by the higher echelons who have the budget for these things, and they’re only hired when they think that their own units can’t hack it on their own. That does not make the units look good at all. No sir.
So, while I’m not 100% sure of a final version of the merc rules, I’m thinking along the lines of a maximum of 1 per game per side, and them not being able to claim any mission goals. As a starting point.