While a fair theory, I would have to disagree. Considering the lack of knowledge of the identities of the the final 5 to the significant 7 I doubt this is the case. While Leoben's ability to see things is quite remarkable, his knowledge of the final Cylon would essentially violate everything we have been told about the 7 in relation to the 5. Additionally, Leoben is incredibly cunning and, as Admiral Adama says, "He mixes lies with truth". I think his comment to Rosalin in S1 is just that, a tactic to spread doubt and fear amongst the higher ups of the fleet.
That being said, apparently there are little hints sprinkled throughout the show so far, that hint to the identity of the final Cylon, so I could certainly see where your theory comes from.