Where can I change the default startup behavior for usb devices? When the popup notice displays showing the device, clicking on it by default opens Dolphin. I want to change that to open Krusader instead.

depends on what you want.

if you want to use krusader in stead of dolphin for all your file management, go to systemsettings » default applications and setup krusader as default filemanager

if you want to use krusader for opening usb's, but keep using dolphin on the side, go to systemsettings » device actions and either change the behavior of the filemanager entries or create a new one.