I think we can only speculate about some of that, but I haven't read anywhere that the Massassi are some kind of spawn from Sith alchemy. I think it's more like our genus (homo), you had several different sub-species walking the Earth at the same time.
Humanoid doesn't exactly mean "look the most like a human", it's more like "2 legs, 2 arms and a head", so the Massassi are also humanoid. "To have fully evolved" is also something that's open to discussion, an ant is great at what it does and the species thrives.
From what I understand, the ones that call themselves Purebred are actually quite deluted genetically, ironic.