Well, I'd figure that since there's no actual alignment between Poison and Barbas' souls, it'd make sense that the energy or whatever he's getting is what powers the form of the new weapons. Technically, I don't actually think there was anything that showed off that Arbiter was Sparda's weapon, and Osiris was Eva's, so what's held in those portraits might really just be some sort of energy that powers more weapon forms. Both Sparda and Eva seem to have meticulously planned out giving him Rebellion and more forms for it to take. I guess it stands to reason that there's probably like...blueprints for other weapons Rebellion can take the form of, as well.
All we really know for sure is that Rebellion can change forms to different things, and it gains a new form when getting some form of energy from either a specific location, or killing something. It could really just be "magic," and not have anything to do with souls.
We gotta remember that just because DMC establishes a universe were demon souls can become Devil Arms doesn't mean DmC's universe follows suit.
And I just wanna say - electric guitar out of Nevan made the least amount of sense in terms of the form of the weapon. Sure the element made sense (electricity and bats, why not), but it being a guitar was the most retarded thing in DMC3 to me >_<
And Alchemist - "calm ur tits," as they say.