Three Person Team, best in my opinion as a GM (I honestly can't say the last time I've played as a player):
Scout: This guy is a hot pilot - maybe not as good as he thinks he is, but still very good. He's an okay mechanic but nothing special but is very good making temporary repairs. His off-time hobbies include drinking, flirting with the ladies, and gambling - his particular vice is actually racing ground vehicles of different types and serves as a decent "wheel man" on the ground, but his interest in driving rapidly drops off when there's nobody to compete against at which point he'll just play cards. He's also a conflicted faithful of some unpopular cause in the Imperium - perhaps he agrees with the aims (but not the methods) of the Ine Givar, he's visited the Consulate and thinks their society is pretty admirable and thinks psionic suppression laws should be abolished, "went native" and fought alongside the locals in some rebellion that the Imperium crushed, etc.
Rogue/Merchant: Grew up in a slum desperately poor, was caught trying to steal by a curmudgeonly merchant who took the player in and taught him (or her) the ropes. Skilled at fast-talking, getting a good price, and generally playing the market. Unfortunately, while his market sense is good, he also wants to be rich and takes risks which have bad habit falling through and has no problems investing money with shady people - criminal organizations and so on. His background as poor also gives him a certain amount of extralegal skills or small vehicle piloting skills. He's an okay "second wrench" mechanic's assistant.
Navy Doctor: A blue-blooded noble who is not in an inheriting position but is in an okay position with his family, except they want him to marry, and he doesn't want to (it may or may not be important why - he might be gay, he might be in an arranged marriage, he's just irresponsible about that, etc.). Despite his name getting him an appointment as a Naval doctor, he's actually a very good doctor and enjoys making people better. He should ideally have some sort of combat-related surprise, like he's some past master at unarmed martial arts, or is a crack shot with rifle, or is a duelling master with a sword. An Imperial Noble, he dislikes corruption, injustice, and cruelty, especially when it is nobles doing it to commonfolk and generally plays the party idealist.
There's certain skills I intentionally leave out, most notably a gee-whiz mechanic. As players, I think it's actually key to have some skill missing, no matter how annoying it is for you as a player. Parties of players that can do it all have a nasty habit of eeling (slipping away) from situations that would make good adventures simply because they have no lacking areas you can exploit. When they do get involved, they tend to be mercenary in the extreme - they don't really have any emotional stake in whatever is happening and are just there in a purely mercenary capacity ("I'm just here to get paid") which gets annoying for any GM who wants games to mean more than just rolling on the cargoes table.
If a party lacks a really skilled mechanic, for instance, yearly repairs, scrounging for repair parts, the failure of ad-hoc repairs, and so on are great ways to strand your players somewhere and make them explore stuff. The concepts allow maximum exposure to possible adventuring: The merchant can get merchantile cargoes and so on, the noble lets the players circulate in the SOC B+ crowd, the scout's has contacts amongst the scouts and similar spacers. There's also secondary motivations for everyone: The scout has his past, the merchant has underworld links, and the noble believes in justice. And there's even tertiary hooks.
