a half-fiend/half-human is a human for the purposes of many things, but its not a truly human its a 'half-breed', same with a paragon human, it is a human for the purposes of feats, etc. but it is no longer only a human, its a paragon human.
an example that actually exists to compare is a half-goblin from Dragonlance, yes its part human and part goblin, but not truly either.
the sculpt spell fighter feat thing is similar, because its trying to add something to a list that is not on an already set list.
i just don't understand the possibility to add to this list, when it's set in stone.
The term serpents includes all of the following creatures from the books published for the D&D game.
the Serpentfolk list actually includes templates, so those templated creatures (which could be pretty much any creature as long as it has the listed template) are included on that list.