They sound less like mercenaries and more like just a group of good guys with guns. 'Mercenary' implies work for pay, and not necessarily for a good cause. If they are going into Arizona just to free slaves, I don't think the term 'mercenary' really fits them, unless they are only doing so because someone is paying them to free the slaves, in which case you should probably elaborate on who and why. 

They seem more like a Rilley's Rangers kind of group - a group of people who have taken it upon themselves to help the people of the wastes out of mostly the kindness of their heart. But, to be honest, it's not really likely anyone would do this kind of work purely out of the kindness of their heart. There has to be something more. Something that makes risking their lives for strangers worth it. They could either be getting paid by wastelanders that appreciate and support their work, with things like food and supplies, maybe shelter, and other necessities besides money (which they may not ask for, but keeps them able to continue operating without being hired). 

Or there has to be a strong reason for their willingness to risk their lives. Maybe a lot of them are former slaves that know how much slaves suffer, can relate, and feel an obligation to free slaves they come across. It could be how they gain new members, too -- by freeing slaves, who then join the group looking for a purpose in life again. The group could also be made up of people with personal grudges against those it targets. People who have lost everything to groups like the Legion, and have no qualms with risking their lives if it means payback.

Whatever the case, it should be more than just "these guys are super nice and have no problem dying to help people" because, let's face it; that's not how most people work. There may be a few people in their ranks like that, but most would need a reason to do what they do.