There is a principle of riot control that says that the number of people a riot-control unit can control is proportional to the size of the unit multiplied by the level of force it is prepared to use.
When I was young enough to spend some of my time at university marching through London carry a placard, the Met Police had taken this to heart by using a technique called “swamp policing”; they cancelled leave, routed officers out of their cosy offices, got reinforcing elements from Thames Valley and other neighbouring forces, and put as many officers on the demo as there were demonstrators — possibly more. The police officers formed neat close-order lines on each side of the march, and it proceeded in an entirely orderly and non-violent manner to Hyde Park Corner, where the only slight glimmer of hostility I witnessed was a red-faced march steward wearing a Labour Party badge yelling at some Hare Krishnas “Stop that f*cking drumming, or I’ll belt ya! People have come here to talk about PEACE!”
At the other extreme, a chap who used to be our family grocer once told me of an incident that occurred in Hong Kong in the late 1930s when he was a member of the Royal Marines detachment of HMS Ramillies (I think it was). Some of the locals were rioting, and the Marines were called upon to deal with it. They were outnumbered hundreds to one, but in the finest traditions of the service, and as required by law, they stood an officer on an orange-box, and got him and an HKP officer to read the riot act, and warn the crowd to disperse, under a hail of miscellaneous missiles the while. After a regulation pause, the crowd having failed to disperse, the RM officer nominated a rioter who looked like a ring-leader; a Marine fired a single aimed round, which hit the ring-leader in the head and killed him instantly; and the Marines then advanced with fixed bayonets to recover the body, while the crowd dispersed in haste. After Ramillies had left Hong Kong, another riot blew up. Ramillies was ordered back to HK, and when the rioters saw her Marine detachment disembark, they dispersed at once.
So, a military unit can certainly deal with a civilian mob of any size — if only it is prepared to use enough violence. The puzzle the rules should put to the player is, how much violence is enough? Too little, it has no effect, and probably the rioters grow bolder. Too much, and you have massive national or international repercussions from a bad decision by a junior commander — consider the broader political effects of Armristsar, Bloody Sunday (either one), or Kent State. In a deliberate and planned riot-control situation you can probably engage in a more or less stately dance of progressive escalation, each side upping the ante until the other folds. Units properly equipped for riot control with have sufficient options for a number of steps along the way — CS gas, water cannon, dick guns — before starting to shoot people. The side that folds should always eventually be the rioters, as it is another principle of riot control that government forces are never seen to retreat; but if the rioters do well, they will have extracted a fairly violent reaction, and possibly got some good pictures to show people to “come and see the oppression inherent in the system”.
If it’s a question of a military unit getting to safety, then doubtless there is no time for the dance of escalation, and one will just have to hope that posturing as if about to inflict extreme violence, say by ordering “fix bayonets”, will do a good enough job of clearing the way without having to bayonet anybody. Of course the military unit should never have got itself in such a position, and there will surely be a court of enquiry about who cocked up bady enough for this to happen.
Yet another principle of riot control is that people fight harder when cornered. So, yes, the miltary unit *will* clear the way if its survival depends on it. With the boot on the other foot, intelligent riot controllers always make sure that the rioters have somewhere to run. Except in the specific case of snatch squads (short-shield men if you have proper riot control gear, Tommies with steel helmets and pick-helves if you don’t) who need to lift a particular individual, you are really not interested in arresting people, at least not today, you just want to clear them off the street. This means that the tactic of “kettling” which the Met Police have been using against demonstrators in recent years is a harmfully stupid one, but it will be fine as long as the people it is used against are well-spoken middle class folks protesting about something they read on Facebook, and not a bunch of street-fighting thugs with iron bars and stabby things.
How one translates all this into rules I admit I have no idea, but I think if you can capture the principles that force times mass equals effect, that using too much force is a very bad thing (much worse than using too little), and that cornered rats fight harder than those that can run away, you will have got a pretty good first-order model of the way these things work.
All the best,