Realistic skins
 

So I already have a some Maxis-style default skin tones. Is there any way to add realistic details without getting new skins? Like an overlay?

I think the overlays will usually only effect the face.

Anyway, I suppose your question belongs in the Where Can I Find section of the Help forum.

There is a mod on Sexy Sims 2 called "Real Body Overlays - Anatomy Bits" for females if that is what you are looking for.
There is also a fix for it just PM me after you get it and I will give you the link.

Since Seasons there are body overlays. Tans and burns for instance. A few modders have created tattoo overlays. I use a male body hair overlay box as well. Maybe that's something OP would like? It's on sexysims (because apparently chest hair is too naughty for MTS).