I like the banner alot but one thing is wrong with your army from fluff perspective and that is that the Emperor had sanctioned against the use of psyckers therefore none of the Pre heresy Legions had Psykers. (exept for the TSons and look what happened to them.)
The Librarian is fine-- the Legions had them, albeit in limited numbers. The Edict of Nikea, where the Emperor censured Magnus for how he proceeded with psychic talent, defined the role and training of psykers in the Legions, and actually mandated that the Legions include Librarians.
The problem with the fluff, however, is the seconding to a Rogue Trader. Rogue Traders are a very post-Heresy thing-- prior to the Heresy, it fell to the Legions to explore the galaxy and reunite humanity. After the Heresy, that still needed to happen, but the decimated Space Marine Legions had to focus on reorganizing and ensuring the continued stability and survival of the Imperium as it existed, so the Rogue Trader charters were created. At this point, the Luna Wolves had become the Sons of Horus, and would have fled to the Eye of Terror following Horus' defeat at Terra.
Space Marines tend not to look favorably upon Rogue Traders, to boot, and tend to view them as petty thugs who have usurped the grand and lofty goals of the Great Crusade from its rightful prosecutors. The only thing that really stops them from coming to blows is the Rogue Trader's charters, which are sanctioned by Terra, and no Space Marine Chapter would dare step on a holy mandate such as that.
I'd decide whether or not these Luna Wolves were part of Horus' expeditionary fleet, or part of another which contained elements of the Luna Wolves Legion; which Grand Company they belong to, and who their Captain is; and when during the Great Crusade the army is supposed to be from. That'll help you piece together the fluff for the army a bit better, and will make the models more accurate representations of the Luna Wolves from established fluff.