Know how I'm sure kubo is a masterclass artist? He drew the thunder kitty yuroichi fight scenes with one hand, matter of fact it got to intense he had to swap hands and even use his teeth to draw. Epic
Rangiku was already a vice-captain prior to Toshiro becoming captain, it's likely the only reason she stayed a vice-captain after toshiro's promotion was due to their familiarity.
Its wild cause zanpakto reflect the users soul .. so like they nearly never are incompatable with their user..... So like iven though it def would match her, she still just said nah i can do better
Rangiku is actually very smart. She just perfers to chill. And not take things seriously. Same with Yoruichi, She mentored soifon. And from the sound of it, Rangiku probably mentored toshiro a Little. Not in fighting clearly. But in other areas.
Technically Rangiku didn't become Toshiros lieutenant. She was already the lieutenant while he was still third seat of the squad. She remained as lieutenant because she was already proven to be trustworthy and competent in her own right to the last captain, and was a comrade of Toshiros by a while at that point.