MacOS – How to add more categories to Mac OS X Finder’s sidebar

First, open the trash can by clicking on it in the Dock.
In the Finder, press Command-shift-G (go to folder) and go to ~/.Trash
In the Finder window for .Trash, click the proxy icon in the title bar of the window and drag it to the sidebar.
You then have a trash folder in your sidebar.

To empty the trash from the sidebar, open Automator and create an application. As the sole action in the application, select "Run AppleScript" and use this AppleScript:

on run {}
  tell application "Finder"
    empty the trash
  end tell
end run

Save the application as "Empty Trash", and drag it to the sidebar.

No. You can rearrange the order of the three you mentioned though - just click and drag to move them.

A possible workaround would be to create a new folder, say "Cloud files", and put it in the sidebar. You could then place aliases of your other folders into that one. This would help trim down the folders in your Favorites section, as well as keep the same quick access to them.