First a time issue: Guardians must have a ghost. Therefore there were no guardians in the Golden Age. The first guardians would have appeared just after the Traveler "died" during the Collapse at the beginning of the Dark Age. If we accept this our guardians could not have been guardians in their past "lives" as we would have died in the late Golden Age or very early in the Dark Age. Basically anytime before the City Age, I guess, as there would have had to been time for the Awoken to be created after fleeing the Collapse and some of them to have "drawn" back to Earth to help form the City then die to be risen a guardians. This raises a couple of questions for me: Do we remember our lives before resurrection? There seems to be some indication in the Exo grimoire cards that we do not. What were we in that past? We could not have been guardians, but we must have had some ability to wield Light even if it was latent. What were we doing near a Cosmodrome when we died? We could not all have been fleeing as any Awoken would have had to be one of the ones that got away or their descendants.