It's a case by case basis, and one important factor is whether or not the things that these writers wrote stayed in continuity.
The "cosmology" is a starting point, a line in the sand that starts our tiering process within a specific narrative sandbox that an author created rather than assuming a full composite, due to the various issues with that.
For the case of Buddy, aka Animal Man, the "narrative relevance" factor may allow him to scale to the entire run, but it depends on what those other authors wrote, and whether or not there's a significant lack of congruence between authors, and then a decision will be have to be made about how to reconcile the incongruence in terms of tiering.
So while Buddy might have narrative relevance to those cosmological concepts that later authors wrote into that run, if those same concepts were completely orphaned outside of Animal Man, it does not follow that we clump all of it together under a "Morrison Cosmology" and start scaling unrelated characters and storylines based on these cosmological elements that never made it out of Animal Man at all, and the only connection is that it's in a storyline Grant used to write. That sort of thing is precisely what we are trying to avoid, scaling characters to cosmologies that they have no actual connection to.
Some cosmic concepts have clear and demonstrable staying power. Limbo started in Animal Man, but Grant used it as recently as 2015 Multiversity. That's an extremely clear case that Limbo has remained an element in the cosmology.
This becomes particularly important when we address characters who are creator beings, beyond creation, transcendent, etc. Putting everything the various Animal Man authors in the 90s wrote into a single "Cosmology" and then scaling every transcendent being above every concept within it doesn't make sense. It's not reasonable to interpret these characters as having dominion over these concepts that only existed in Animal Man, and that the authors didn't consider or weren't aware of.
And as an aside, Morrison in particular has an extreme fascination with myths, physics, and Cosmology, but he is horrendously inconsistent and tends to play fast and loose with continuity, so once the discussion thread about specific splits is held, in my opinion serious caution needs to be used in deciding what scales to what, since his work is a whole barrel of monkeys unto itself in terms of contradictions.
But the specific details will be ironed out later on, and my assessments here aren't authoritative, but this is the general idea being employed. If I got something wrong, any of the other staff and project members can chime in to help better communicate it.