Castor and Pollux are the twins from the constellation Gemini, the twins. On the opposite side of the star calendar is Sagittarius, the Archer.
Castor and Pollux's last name is Troy, referencing the Trojan Horse, which is effectively what the face transplant surgery turns Castor and Archer into.
The blood types for the two main characters (i.e. Sean Archer and Castor Troy) are mentioned in the movie, and at a subtle level they reflect their antagonistic nature. Archer's Blood type is O negative, while Troy's is AB positive, these two blood types are as different as you can get. O negative (the universal donor), reflects Archer's role as a policeman dedicated to serving the community, while Troy's AB positive (the universal recipient) suggests someone who takes from society without giving anything back.
If Troy and Archer have differing blood types and this fact is a plot point in getting Archer's wife to realize Archer is Troy, how come Archer's body didn't reject Troy's face when it was put on him?
Anti-rejection drugs. These days people that get Real Life facial transplants need to take them for basically the rest of their lives.
Same applies to any transplant really.
How does his wife not notice his penis is different? Something brought up by many critics, including Roger Ebert.
It's implied that the couple haven't been having sexual relations for a while. Beyond that they two may not have that different a penis. It's not exactly a one a kind part.
An early draft of the script had a line stating that Castor and Archer had virtually identical body types and, yes, penis sizes. While the line isn't in the film, I imagine the idea must have still been in mind.