Helo is ruled out because the producers have told us that Hera is a human-cylon hybrid. This is not to say that they couldn’t allow S7 Cylons and F5 Cylons to breed, even though two S7 can’t breed, because F5 are entirely different. However, they have told us that Hera is half human. Thus Helo is not.

And the F5, remember, have no special loyalties to the S7. Helo killed those Cylons to stop a genocide, and that could be anybody’s motives.

He and Cally are actually the only two truly ruled out!