I mentioned WoW because there are various playable races in that that wouldn't work if they started at the same time as other races. Pandaren start at the beginning of the Mists of Pandaria expac, while Highmountain Tauren/Nightborne Elves start at the beginning of the Legion expac, and Zandalari Trolls and some others start in Battle for Azeroth.
So again, no reason it can't work lorewise. Just have them start later. Also, explain how Sith Jedi Knights/Consulars work lorewise but playable Voss somehow don't. Seems inconsistent.
Sucks about the human skeletons requirement though. That explanation at least is a good one