I had a pretty similar problem to you last summer when I was using some generic dell crappy mouse for around 10 hours a day, although it was my little finger on my right hand that had the problem. I found that it was because the mouse was just too small, width and height wise and also too light to what I was used to before hand.
I changed to a MX1000 and have never had any problems at all since, but it did take a while to get used to the shape of the thing, but now it feels really strange using something different. Its just one of those things that will feel weird at first but then become so natural you wont even notice it.
I think getting a different mouse is probably your best option as it will force you to change the way you hold the thing. I now hold it with my index finger pretty much parallel to the side of the mouse resting on the left button, my middle finger resting on the other side of the scroll on the right button and my other two fingers off the side with the tips slightly resting on the desk. My thumb rests in the grove below the buttons. I guess its not that dissimilar to in that link you put up earlier but the whole of my hand rests on the mouse which I find much more natural and comfortable then not doing it (thats where I think my problem came from).
Thinking back I think I used to hold a mouse like you described but got my pains elsewhere, and I guess getting the pain when I did was actually a good thing as it made me change how I was holding it before it did some major damage.