It'd be easier starting with the weakest iteration and working your way up from there.
Presumably the Fossil Lord will be the strongest of the four, analogous to a Greater Demon, powerful Vampire, or maybe even a
Nightwalker.
I'd suggest deciding on an appropriate Challenge Ranking and role for the Fledgling/Haunting/Flesh Master/Fossil Lord before starting to decide on the numbers.
While you're at that, try to come up with a shctick that distinguishes the monster from other menaces or otherwise makes it memorable/creepy.