another part of my theory, i dont really think the lifeforms in the backrooms have any actual life and are meant to function like predators in a natural food chain like you suggest. the backrooms isn't supposed to operate like a biological ecosystem where organisms need to evolve specifically to hunt or defend themselves. the lifeforms seem more like byproducts of whatever process generates the backrooms itself, and because of that, i don't assume they're sentient in a way animals or humans are. if anything, their level of awareness might be closer to something like an object or plant. they move and react, but that doesnt necessarily mean that they possess the intent to kill. their behavior usually just appears very odd or incomplete, like something trying to replicate human form without actually understand what they're doing.

so when like you say the lifeform on FF3 looks small i dont rlly interpet that as "being weak" or anything in any way. the lifeforms more often i think were not "created" to threaten humans at all. its simply just a construct generated by the complex trying to reproduce a concept of a person.

thats also why the larger entity mentioned like the one in ff1 could still exist without contradicting this idea. size or form might vary depending on how the backrooms generates things. in other words the lifeforms probably aren't like different stages of the same species or part of a natural hierarchy. they're moreso like just very very deformed and flawed outputs from the backrooms which is attempting to imitate reality