I use both my left and wight hand for work, and I am strictly right-handed in games. I play equally well (or bad depending on the point of view) with all grips, but I definitely prefer a relaxed palm grip, with partial contact. My hand width is almost 10.5 cm and the 60% rule yields 6.3 cm. It is a fairly large value for mouse width, and does not match the width of any of the mice I own, supporting the hypothesis that the permanent swelling and pain of my little finger could be caused by my attempts to keep it in contact with the right side of the mouse. Moreover, although I own mice of all sizes, all my mice either are not tall enough, or do not give enough support for the right half of my hand, thereby increasing the perceived "excess length" of my little finger.
I don't understand why ergonomic shapes are, in their right part, just shapes which drastically taper from bottom to top, and with a slight outward right curve along the major axis. That design is way too cautious to give some support to players with a hand width significantly greater than 10.
In one of his reviews, MM530, RJN says the mouse "goes out with a rest for the little finger" and that is makes sense for "people who haven't adjusted their grip to go further inward". Well I definitely have failed to do that for all my mice, and, while I am not surprised the FK2 caused my little finger to hurt, I really hoped that buying larger mice would have solved the problem but it wasn't the case.
Last item in the list of failures is the otherwise exceptionally good Rival 600, which would be the perfect mouse for me if it gave me more support for the right half of my hand; being my main game mouse now, I have to choose between bending my little finger more, or letting it rub against the mat. Either choice does not help me reduce my inflammation. Unless I hold the mouse slightly sideways; that's a cheap way of making a mouse feel larger over all, extend my little finger more, and raise its tip from the mat. Does this mean that is the way to hold a mouse? I don't think so. Interestingly, the Deathadder shape automatically promotes that type of nonorthogonal grip and in fact it is probably the most comfortable mouse I own. Unfortunately I bought the left handed version for work only and I don't know if the right-handed version would stand the test of long gaming sessions.
Mice I haven't tried are the previous iterations of the Rival, the above mentioned MM530/531, and the Mamba Elite. And, of course, the MMO/RPG shapes like Spawn/MM520 and Naga. These can however be rather flat and short.
If you have experienced the same problems I have, I'd be very happy to read your comments and suggestions.