SSBNs stay submerged for 6 months at a time. I don't think the difference between 6 and 8 is enough to matter.
Umm, no. I served on USS Kamehameha. SSBN 642. Two months and change out, switch crews, repeat forever.
Many subs have had smaller crews, and nearly all of your interactions are with a few people at your work station.
While virtually all subs had smaller crews than modern SSBN's, they seldom had crews fewer than 30 or so. Notable exceptions being an assortment of "minisubs" used at various points in WW2, all of which spent a couple days underway at a time.
And it might surprise you to know that you seldom socialize all that much with they guys at your workstation. When you're working, you're too busy for much in the way of social interactions. You interact socially with the guys on the messdeck during meals and movies (when you can stay awake to watch a movie).
Note that one of the biggest problems with a trip to Mars is likely to be boredom. Six of you in a freefall can. No course changes, no repairs, not much in the way of science to do till arrival.
Mind you, a lot of that can be fixed by sending a bigger expedition - 60 guys plus instrumentation and such for doing some decent science while underway, that sort of thing....
But there is another huge psychological consideration that makes a sub much more like a space flight: You can't quit.
Now this I can't argue with. A good point. Note that this makes the test even more (potentially) useful. If the guys in the dome can't handle it in Easy Mode, sure as shooting it won't work for a Real Mars mission....