The identities of all Cylons were revealed by the end of BSG. The whole story about them, who did what, who was created by who, etc. All the slots are taken and the old man simply never was a Cylon. It would actually break continuity if they now started claiming that he was.
The reason he was able to fight Leoben was that Leoben was already half dead as the radiation present at the site of Ragnar Anchorage was lethal to Cylons.
The only Cylon we never saw was Daniel. From what we were told, he was a sensitive soul, liked to paint, and was close to Ellen, which is partly why Cavil murdered him and did something to the fluid in which all of his clone bodies were stored, effectively and permanently destroying that model forever. Even if one survived, he couldn't have been the old man. For one, I can't really see him as a sensitive artistic soul or being best chums with Ellen, but more importantly he would have been recognized by the final 5 once their memories kicked in. He wasn't.
Seriously, saying that the whole colonial fleet was run by Cylons (both the admiral and his xo? Come on..) would be ridiculous and introduce quite a lot of inconsistencies.
Furthermore, William Adama's file we see in the BSG episode "Hero" states his mother's name is "Evelyn". Willie Adama's mother's name was Shannon, and Evelyn and Joseph just got together in the last few Caprica episodes. This would suggest they are going to have a kid of their own somewhere down the line and name him William again.
ZOe was a Cylon, and wasn't included in the Cylon history, was she?
The 12 consist of the "orignal 5" from Earth and the 7 they created for the Colonial Centurions in order to broker peace during the Colonial Cylon war. The fact that work on skin jobs has been done on Caprica prior to the war means there could have been others that the 12 didn't ever know about. Plus, of course, Daniel, who, despite massive argument to the contrary, seems almost certainly to have been Starbuck's father. That seems to contradict continuity, in that Cylons aren't supposed to be fertile, but maybe the suggestion is that a human and Cylon together works... But, then again, how could they have populated Earth with skin jobs if they had always been infertile? Massive factories with assembly lines of duplicates? Maybe it's something to do with resurrection - they could have one or the other, but not both?
As for having too many Cylons within the Colonial Fleet, why not? If Tigh, Tyrol and Boomer could get in (and, later, Anders), and another could be the President's aide, why not another? It's not as if the Fleet had an allocated Cylon infiltrator quota...
I didn't know about the mothers' names, but BSG hasn't always been good about continuity - just look at what they did to Lee and Starbuck's characters.
I said I haven't seen the episodes yet, so I can't make any interpretations. That said, no-one has seen the ending yet, so no-one can be definitive about this either. All I'm saying is that the theory is not impossible or even unreasonable.
