I would think that was possible. when you start the install, select 'custom', then deselect all the apps you dont want to install, leave only Outlook. that will then only install Outlook. I have upgraded in the past, so I would assume that the XP version would automaticlly search for other versions, and if it finds them, then upgrade them. give it a try.
not sure about downgrading, you might need to uninstall first, and depending on the OS you are using, you might need to save the outlook files first, then uninstall, then install. and then put the saved outlook files back into place. again, not sure about that.