From what I've seen, they do not. By the time it matters, though, you should be able to solo any mission you can get at this point in development. I'm sure things will change in the future, however.

also they are single run guys but you can call them for each run. the cost is tacked on after you make your run and there is a percentage as well as a flat fee. no they don't improve but you can get stronger ones. rember to TALK to them you don't have to higher them just talk to them. then if you need em you can call them