Get back to topic that is about mutie mercenaries remember?
Excellent idea.
So, what we found so far: raising price for muties won't work because there's a money leak somewhere.
Raising Charisma check on Muties won't do much sense, either.
So, i would suggest that Mutants won't attack players sometimes on certain conditions.
Like a single unarmed player will be told to GTFO and 10 seconds to comply, just because Marcus says so. A trained minigun however, ticking (whispering) explosive in inventory or a heavily armed party would get you a lead hurricane.
Before you say, a player could sport his gun behind their backs, gather single fighters into the city past the cordon, so be it.
Let them temp their fate.
Someone keeping his eyes open would notice it and react accordingly. If he's away - his fault.
As for attacks with mutants, i say Avalanche effect against overkill. First, only 2 at once attacks, if it results in death, 4 storms the front and so on.
Make them really self-deciding.
Deciding that they don't want to be your PERSONAL ARMY, if there's too much of their own kin around.
Imagine, there's a war and everybody goes to pub.