Generally, such an unstable guy should probably never have been made an Astartes, much less a Librarian. The Librarians that look into the recruits' minds for psychic potential would probably have notice the presence of other personalities, or at the very least the turbulence in his brain.
It might work if he developed these personalities during his time as a Librarian, maybe through some trauma, decades of dwelling on the powers of the warp, etc. You'll probably have to do a lot of research into how and when "other personalities" develop (especially since there is a lot of debate in the medical community as to the validity of dissociative identity disorder and its effects), and to accommodate those to the situation of the Librarian.
Still, though, I imagine his battle-brothers proooobably wouldn't exactly be comfortable with him around if they found out.