Firms typically use as much violence as is economical. A company that uses excessive force would incur high costs and have little cooperation from the community and competitors. Reputation would be a factor as a firm that becomes a crime syndicate would have to deal with a hostile public. An oligopoly by a cartel is more likely than a monopoly but even they would have a hard time competing with innovators and voluntary militias. Another benefit to polycentric law (which has much better historical examples than Mexico) would be less incentive for people to be incarcerated for victim-less crimes, that alone would mean most of today's prison population would be free and those resources would be used to protect property rights instead of enriching drug cartels and gangs.