There are a few different things to address here.
First off, Edward and Alphonse are fully human. This is actually addressed when Alphonse first finds out about his dad's origins. Essentially, he contains a philosopher's stone within himself, but he is still biologically human.
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Second, remember that all humans are philosopher's stones. Edward (kind of) states this in episode 41, and says that his life energy is identical to that of a philosopher's stone, and he's right. All humans are simply philosopher's stones containing exactly one soul.
Third, it is because of the above points that Father approached Edward. It wasn't because he'd get a lot of energy, like from a true, isolated stone (such as one of Kimblee's), but rather because Edward had one soul of energy within him. Father was desperate.