Metallia is inhuman enough as an entity that I honestly doubt she'd have a recognizably human "past life" to refer back to.
Would you mind expanding on that? Lack of familiarity with the source material strikes again.
Strictly speaking in the source material, 3rd arc of the first season (3rd General too) deals with Zoicite hunting down seven Rainbow Crystals that were used to seal Seven Great Youma, powerful servants of Metallia.
As it happens the 7GY are reincarnated as a bunch of people in modern world, namely - an arcade game enthusiast with telekinesis, a school boy with future sight (the one guy who had seen Zoicite coming), a catholic priest, a college student (I
think it was archaeology), an artist (whose paintings are based on Silver Millenium era), a Shinto priest and a really obese cat of heroic disposition. The Rainbow Crystals are extracted from them which turns them into a Youma. Until Sailor Moon uses magic to turn them back.
The climax of the arc has the Rainbow Crystals combine to recreate the Great McGuffin of the SM franchise, Illusionary Silver Crystal aka Maboroshi no Ginzuishou.
Incidentally weren't Rainbow Crystals mentioned in the Precure part of this crossover?
EDIT: I think the big deal was that Rainbow Crystals were initially Fragments of Silver Crystal and yes, could be combined to recreate it. Which may or may not be the reason 7GY were reincarnated - we may assume they were marked for preferential treatment by the Rainbow Crystals as opposed to every random Youma who were not so privileged. We don't know either way.
EDIT2: come to think of it 7GY are not the only people to be reborn with magical jewels in their souls - Kagome (from
Inuyasha) is also one, for Shikon no Tama. Amusing coincidence that.
And no, Senshi don't count because magical jewels
are their souls.