2R1B is designed to work well with larger groups, so it's difficult to make it work well in the 6-8 player space. I assume you're already following the rules that for 10 or fewer players you only have 3 rounds and cannot colour share.
Beyond that, the main considerations are to make card sharing risky while still being necessary, and encouraging deals, so I would suggest that with 6 players you might want to have President, Bomber and 4 Grey roles. As to which Greys, potentially something like Ahab+Moby, or Sniper+Target+Decoy, where it's highly beneficial to know who the other Greys are without giving away your own role, which might involve trying to find the President or Bomber and making a deal with them. Romeo+Juliet is also good, although they're actively trying to find each other so they're probably more open to publically revealing.
Burying a card can also make things interesting, although with only 6 players if you're burying a card then you either need to ensure that President and Bomber aren't buried, or you play with President's Daughter and Martyr plus 3 Grey cards, and try to make sure that those three don't cause issues with burying (either by only burying one of the four Red/Blue cards, or picking Greys that don't depend on each other). There's still an issue there, though, that you'll wind up with one team who has 2 players against one who may have only 1, which gets a bit unfair. You could make one of the Greys the Amnesiac/Drunk instead, who gains the role of the buried card in the last round, but that just means that you're really playing with the normal 6 cards but someone doesn't know how to play until round 3 (unless they can somehow divine their card earlier), which may wind up being even less fun.
The print-and-play rules include a number of suggested sets that go down to 6 players - the first group are more tutorial ones, but from page 21 (of the v4.0 rules) onwards there are several that are more designed to make for interesting decisions around card sharing and hostage trading. For example:
2 Rooms, 1 Boom and a Gunshot
- Hot Potato
This one has a lot of potential for crazy shenanigans - the Hot Potato always loses, but anyone who card shares with it trades roles, so even if the President has worked out who the Bomber is, or the Sniper their Target, things can still go awry if they don't keep an eye on them.
6 Player Instant Death
- Tuesday Knight/Dr Boom
In this one, the President and the Bomber both know they need to card share with the Doctor and Engineer respectively, or risk their team losing. At the same time, if they share with Dr Boom or Tuesday Knight, it's an instant loss for them and a win for their opponents. So Dr Boom and Tuesday Knight will probably be claiming to be the Doctor and the Engineer. But then if that role is in the same room, they can counter-claim. Or maybe knowing that that's expected they'll try an even trickier tactic. Maybe the Engineer can claim Bomber to lure out Tuesday Knight - or claim President to try to find a teammate in Dr Boom (or find the Doctor).