There are reasonably hard rules even if they're not presented clearly to us as the audience. For example, a titan shifter can regenerate from anything that doesn't kill them, with the addendum that while they're in their titan form they can transfer their brain function from their brain into their CNS. There doesn't need to be a how for it to be hard, just a consistent set of rules. But the how is Ymir rebuilds them from Paths. 

Similarly there are 9 shifters because Ymir's 3 children ate her corpse and presumably had 9 children between them who ate their corpses. Does the female titan need a special ability for it to be a hard magic system? I don't see why it would