Lucky, snagging that Battlebarge. Good luck on acquiring the rest of the fleet.
The Librarian is shaping up to be very impressive... You were thinking about parchments over his right shoulder. But seeing how, as a Librarian, he is already used to standing apart from his brothers, wouldn't he use the stark black expanse of his shoulderpad as a shield and deterrent/challenge? Discouraging his fellow Deathwatch from approaching him even more? Enforcing his self-appointed isolation through it?
If you think the pad would be too bare like that, you could consider either putting a black shield in relief on it, or dotting the edge with Deathwatch/Inquisitional service or loyalty markings? After all this is a marine who has given himself mind, body and soul to the Deathwatch, and through that, the Ordo Xenos. He will have seen a lot of action, maybe even be partially initiated in the Ordo Xenos itself, and be marked for it. Little stuff on the edges will work to emphasize the emptiness of the pad while adding detail.
As for the torsos, you could divide them as works/doesn't work, but there are many different body types and builds. This goes for humans, and probably for marines as well... And people tend to ascribe certain temperaments to certain body types. That terminator torso on top of that wide stance, depending on the arms and weapons you give him, could either show the marine in question to be a sort of immovable object, dour, stubborn and heavy handed, or a freight train, once he gets moving he will run roughshod through everything. Either way, his approach isn't about speed or finesse or skill, but sheer brute strength.
What I'm trying to say, in a roundabout way, is to look at the sort of character you want to portray and find the torso/bodyshape that suits it best, for that one instance. Marines are individuals, not clone troopers. Use that to your advantage.