In the root of my application I have 5 named "slots" (layers) that I want content to appear in. Should I be using Canvas, Group or something else for each of these slots? I don't need any kind of automatic layout inside these slots.
Use Group. You can plug a custom layout into it if you wish. Here's a good preso on what's new with Flex 4 Lists and Layouts:
http://tv.adobe.com/watch/max-2009-develop/a-deep-dive-into-flex-4-lists-and-layouts/