A deleted scene in that one episode where Helo suffocates the cylons has Gaeta being promoted to XO when Helo refuses the mission. So since Helo is needed as a raptor pilot for the passage, that means Gaeta gets promoted to XO again until Tigh shows up.

Huh, I never thought about the whole radiation thing before. Though also, the cylons are able to survive ok on post-nuclear Caprica. Guess they didn't think too hard on that one.