The Buddy uses an 8mm thread. You could potentially use the 9mm (assuming it's a pass through mount) and make sure it's good and tight if they don't have a mount available in 8mm.
On thinking on the subject a little more, I think that 9mm is the size that you're looking for. They don't seem to have either 8mm or 10mm (the two most common mirror stalk sizes). If it's a pass through design, then you want it larger than the 8mm threaded shaft of the mirror stalks, in which case the 9mm is the ideal size.
You should be fine with the 9mm mount. Be sure to give us some pics though.
Also, I don't think that a lock washer is totally necessary, the locking nut on the mirror stalk will take care of that just fine and space will be a little tight with the added height of the ram mount bracket.