C&G and Redcaps as Merchants

City and Guild, p.108, states: "The Redcap Virtue may substitute for any of these Virtues [Merchant, Merchant Venturer, Factor and Capo]. Redcaps change roles at the whim of their House." Is it the StoryGuide or the player who gets to choose whether a Redcap character is a powerful Capo or a lowly peddler?

The player, within reason. If the SG doesn't want the story going "that" way, talk to the player, but otherwise, companions are supposed to be important people. Just not as powerful as a mage....

The dynamics of every group of players is different, but in theory a "Troupe" should make group decisions, and any player should make decisions for the best story, not just to best advance their own character(s) (again, in theory).

So long as the Player isn't redefining weekly - or isn't without a good explanation - and isn't doing it simply to abuse that phrase, there's no reason it shouldn't be (in large part) their choice, the same as any other chargen decision.