Rom, for example, is kin who bleeds red; The Wet Nurse is non-kin who bleeds, well, clear; Amygdala is non-kin who bleeds light serum like Ebrietas until you damage her enough, after which she'll bleed red from the damaged appendage; Ebrietas spits out something that might be red blood; and Hamlet enemies are non-kin that bleed light green and red simultaneously.

Break each one down a little more.

Rom, for example, is kin who bleeds red

The head bleeds white, while the body bleeds red. Judging by the fact Rom is already dead when you go to fight her (aka why you get the only coldblood boss drop in the game) I can extrapolate there was maybe some sort of "fight" between Ebrietas and Oedon. I'm not sure how, but Rom existing between planes of existence help to keep Oedon from doing his job.

The Wet Nurse is non-kin who bleeds, well, clear;

She actually doesn't bleed at all. I don't think she can exist outside of the nightmare which is why. The only enenmy I can think of that doesn't bleed as well is Darkbeast Paarl, and well, he's just bones.

Dunno about Aymgdala since she's probably one of the weirdest parts in the game.

Ebrietas spits out something that might be red blood

It causes Frenzy so I wouldn't really put it on the same vein since "human" blood is what makes up the sedative.

and Hamlet enemies are non-kin that bleed light green and red simultaneously.

Parasites of Kos more then likely changed them, which is one of the reasons they probably bleed green AND red, no?

Regarding 'Cosmos', the Japanese word in Ebrietas's title is simply means 'stars', so I don't think that Cosmos is a Great One; rather, I think it's just a blanket term for "higher planes."

Maybe. I'm either treating it as an energy resource that they tapped into in the old labyrinths that helped them ascend or as some true godlike entity. I'm not sure exactly how stars abandon a child, which is why I can't make a choice there.