"Pairing" is not required. In Android, installed apps declare what they can do (email stuff, view pictures, select files, etc.) and when an app needs a service it asks Android for a list of apps that have declared their ability to do so. So, when MailDroid wants to save a file, it gets a list of candidate apps from Android, and, in my case it only shows OI File Manager and nothing else.

So, go to SlideMe.org and find and install the OI File Manager app and you should be good to go.

As for other mail clients, you can't actually delete/uninstall them as they're part of the firmware. You can remove the standard mail client from the bottom of the LCD screen by dragging it to the Garbage can icon that appears when you press and hold it, and then you can drag MailDroid into the empty slot (which I, too, have done).