My top three would be Aquarius/Scorpio and Virgo. I'm glad to hear there's no canonical evidence for Cap, because, um, no way. But to me he seems way too organised and... self sufficient to be a Sag, and he doesn't seem to like people as much as Sags generally do. I mean, he doesn't dislike people, but he's quite happy being on his own. He's not really a fire sign to me. I'd say maybe Scorp w/ Aq or Virgo Asc, or Aq w/ Scorp Moon and maybe Virgo Asc. Or even a Sag asc, which could possibly account for some of the similarities you point out above (OTOH, I am not sure that the above reading of a Sag is one I'd necessarily agree with either).

Oh, and he's too honest to be a Pisces. *g* They are dual fish and they like to SWIM AWAY FROM TROUBLE.