I think skin details do go on all slots, like tattooes.
I have eyelashes that are "skin detail", which I use for sims that should have naturally longer or thicker lashes, but also use eyelashes from other categories (eyeliner, glasses, earrings, etc...) for eyelashes I want to use as "mascara" for evening and formal looks.
Likewise, I prefer using horns listed at "skin detail" for monsters that should have them on each look.
That being said, it sure would be nice if there was a checkbox to use the same makeup or clothing for each look. My brother usually puts the same outfit on each look except swimming and sleep, so he has to go through and manually put the same outfit on each one. Even if I put a different oufit on each look, I'm still going to have to change every look, because I don't usually like how the game dresses my sims.