If the rumor about Han and Chewie in control of an OT era Star Destroyer is true, then they either took smuggling to a whole new level, (ala Crimson Jack in the old Marvel run) or they're part of some post rebellion fleet.
Aside from some of the top military leaders, the rebel alliance never had much of a dress code anyway. (They're rebels!) And even Indiana Jones doesn't wear the jacket and the whip all the time. ;)