Rahab summon might be useful, but neither are cut for main eidolon and these days their stats are also considered kinda low, so they aren't really fit for sub, either.

As for the weapons, Medusa's is pure defender, not much reason to bother with it. You should definitely max Rahab's weapon, however.
And the SR weapons can also be ignored.

You will probably have more than enough time/AP to do that single weapon, so you might want to try farming Medusa weapon or farm books, but that's up to you, I guess.