... for Missions? They can't get a license. Period, full stop. Missions work off the standard SR.
If you want them to have access and license to use heavy (Forbidden) weaponry, then they get it from the corporate quartermaster, they sign for it, for the bullets, and they account for every round fired when they return the weapon. And they'd better return the weapon - or enough of it for the QM to agree that it was screwed. If this is the way you're going to go, then don't bother with the merc route, because they're already signing up as limited-authorized independent contractors. A Sinner NQ would help, but it isn't absolutely necessary. And they definitely don't need to form a merc corporation.
The merc corporation - which I'd benchmark the bond at a minimum of five million nuyen, plus an additional million nuyen per registered team member - is if they want to own the weapons themselves, and be able to show some rent-a-cop that searches their place that they are what's called in the biz an 'end-user certificate', issued by the Merc Board in Lisbon and legal in any country that employs mercenaries - which is ALMOST all of them. I do not, however, think that the UCAS is one. Salish-Sidhe, on the other hand, yes. It does not, however, allow them to carry outside a war zone.