Corporate Security forces would not be considered "Mercenaries", they'd be "security guards", hired as regular employees by the corporation, probably with some serious restrictions by the OA to insure that they don't pose a direct military threat to the planetary government. That most likely means a lance or two of tanks and some foot sloggers, possibly with one or two Battlemechs at most to add some "teeth" to the unit if they can get permission in the form of a local waiver. As pointed out, any 'Mech units would likely be restricted to corporate property, or perhaps only ICE-powered security 'Mechs might be allowed.
The legal shades of difference between "Mercenary", "Militia", and "Security" need to be hashed out in detail by lawyers over a period of time, in order to define (then test and redefine) what falls into which category, and even then, many lawyers have been known to leave themselves "loopholes" for various reasons (either good or bad). In the absence of that process, a little bit of flexibility in the rules is probably OK.