I have a Theory About
Brontosauruses BSG.
I submit the notion that the skinjob Cylons are cloned descendants of the real humans who colonized the galaxy long ago, just before the Big One back on Earth. Because they are clones, they cannot reproduce the old-fashioned way. They may have developed cybernetic or nanotech implants as well, giving rise to the notion that they are cyborgs. The Centurion and Raider models are more like robot servants with biological components.
The so-called humans, OTOH, are an
artificial yet totally biological race/species/whatever that were manufactured by the clones/Cylons to fill out the populations, and the Cylon bio-techs who developed them managed to figure out how they could reproduce on their own, in a natural way, without the aid of a resurrection facility. Hence the diversity of appearance.
About the "final 5th Cylon" - don't we already have two candidates, in Kara Thrace who (presumably) found her own body; and Ellen Tigh, because she was remembered by Saul as appearing in one of his earlier lives? Come to think of it, I thought I had read somewhere that Ellen was confirmed as the fifth Cylon. Which makes me wonder about Kara.