Not all demons were part human only demons native to our dimension I am not entirely certain if there is a canonical list of which demons were native though.
Not all demons were capable of hybridization with humans, most instances of demonic reproduction we see someone or something is about to die a horrible horrible death when a fully demon creature rips its way from their body, or they were going to be eaten after being used to fertilize eggs.
It is likely that different species of demons cannot hybridize even if they both have human blood does not mean their demon blood would mix as far as I know there are no demon/demon hybrids depicted in either show
EDIT: Each half demon we see is a somewhat unique individual so we have no way to compare how their demonic traits are normally passed it is also important to note in the real world hybrid expression can change depending on the gestating species. A quote from the buffy wiki "A half-demon would be something entirely new in each case" is probably the closest one could get to an answer.