I doubt it would work. Both shows deal with mythology but in very different ways. Mars would be technically a villain from the pov of the saints.
That can be easily solved by knowing two background facts:
- We don't know when the Silver Millennium happened, except that it was before the invention of (modern) writing and somehow some knowledge filtered down so that Greeks and Romans knew of their people as divinities.
- Saint Seiya's gods are actually humans who awakened the Eight Sense (or even the Ninth in some cases), either born from normal people or "gods", who predate the fall of Atlantis (destroyed by Athena and her Saints during the first war against Poseidon) and of Mu (destroyed after the above, unclear if it was a natural catastrophe or Athena dealing with the rebellious Cloth makers that created the Black Cloths).
If their universes are fused, it would be easy to have the Silver Millennium predate
Saint Seiya's gods, or even have the earliest "gods" being Silver Millennium survivors, who took the names of the Silver Millennium people (the exception would be Chronos, the god of time (not Kronos the King of Titans, they are two different divinities who were later fused, and in
Saint Seiya they're shown to be different individuals), as he's ancient!Sailor Pluto's father).
It could also be used to have the two parts get in conflict: Athena and her Sanctuary have ruled the world for millennia (with a very hands-off approach), and Sailor Moon, by existing, is an enormous danger to that. Or maybe Sailor Moon goes after Zeus for his reaction to Hades' death. Whatever happens, the end result would be Crystal Tokyo, of course.