I'd prefer the classifications not to go along the lines of D&D, ta.
Shadowrun came up with the concept of character roles within the group based on the tasks each performs: CQB, infiltration, legwork, the face and so on. The face meets the Mr Johnson to arrange the mission and the terms, the legwork guy sniffs around for useful data concerning the mission (and checks up on the Mr Johnson wherever possible), the infiltration guy sneaks into the target area and opens the door for the CQB - or the infiltrator doubles as the CQB guy, in case he needs to bust a few heads on the way in or out, and so on.
A versatile group is one where a team has experience of multiple roles, not necessarily multiple skill sets - though an overlap of skills is highly desirable. Your Marine types might have experience of tinkering with a computer and injecting intrusion software into a terminal so he could take on the technician role for a mission; or he could be chosen to infiltrate a ship based on his previous experience of serving on board vessels of that same class, despite the fact that his Stealth-1 is nowhere near as good as the willowy team ninja's Stealth-3.
And just about every character must know how to handle themselves in a hostile environment, so no "fighter class." Everyone ought to be a fighter in
Traveller.
In truth, it's not so much the individuals' skills that count. It's the ability of the team to work together to get the job done that earns them their reputation and assures them the jobs with the big money.