John takes Paradox slime from Nana, Bro, Mom, and Grandpa.

Slime is used to create John, Dave, Rose, and Jade, as well as Paradox Clones of Nana, Bro, Mom, and Grandpa.

In the original, Pre Scratch Timeline, the Paradox Clones of Nana, Bro, Mom, and Grandpa were sent back in time and became the original Nana, Bro, Mom, and Granpa, thus making their existance a stable timeloop.

All Paradox Clones go back in time to become the original, baring a Scratch.

So you could not use Paradox slime and ectobiology to create perfect copies of people from human history,because they would inevitably go back in time and become the original.

That leaves hybrid clones(combining traits from the originals), and imperfect mutant clones.Even the hybrid clones are iffy, because the hybrid clones from a session created batch of babies are also sent back in time to become the players of the game