Graviton Mercenaries and Nymphs are clearly fantasy creatures and nothing in either of them says they are human. Crusader, who is basically a knight, is a historical human creature. Sure anything could be inside that armor, but when you see the image and read the name, I think everyone assumes it's a human.
I agree that nymphs are mythical creatures, physically resembling humans, but not at all human in nature. As for graviton mercenaries... I don't know any fantasy or mythical race called "Gravitons", so I don't think they are "clearly" fantasy creatures. Yes, of course they can be, but it's much less obvious than in nymphs case.
And 'mercenary' is also a job that in our world is performed by humans only, just as crusader, knight, archer and a lot of other 'jobs' that in fantasy can be performed by other races.
Not to mention the fact that (at least for me) Graviton Mercenaries look like space marines in a futuristic armor, and in all games I know space marines were mostly humans. Looking at Armagio (who resembles a mecha, a giant robot seen both in mangas and in science fiction) only strengthens my feel that Gravity seems the most human of all elements.
Of course it's only how I feel it, based on my experiences.
I simply feel that humans do not fit into Elements world, but concepts like 'knight', 'archer', 'crusader' etc. can easily fit as long as we make it clear these are not humans. And if humans are to appear in Elements, I'd either join them with Gravity, or I'd say some gravity art should be remade if they are to be 'neutral'.