The physical appearance of the Stockwell Cylons is based on the inventor of the original Cylons. The other human Cylons are all based on members of his family/people he knows, etc.

The Hybrids = his ex-wife
Starbuck* = his daughter
Anders = his son
Leoben = his daughter's boyfriend
Chief and Sharon = a couple who were friends of the family
Kevin Spacey Cylon = his lawyer
etc etc

* that said, Starbuck probably isn't the final Cylon