I just soloed several of these guys as BST/NIN (they totally ate me up as /WHM). In 3 successful kills, I got 2 Giant Scales. So the Giant Scale isn't a 100% drop. But unless I was extremely lucky, the drop rate for it seems to be quite high.
Edit (10/6/07): I used my last 2 scales on a CoP 2-5 attempt that wiped, so I went to farm some more scales. Today, I got 2 Giant Scales in 6 kills. They didn't come as fast as my first farming session. But overall, I still got 4 Scales in 9 mobs, which is a pretty high drop rate (close to 50%).
Edited, Oct 6th 2007 12:27am by svlyons