upps,
In answer to the first part of your question, that holder does not fit the Canon P. I use the "M-Grip" by Photo Equip, it's an early version of what they made for Leica M cameras, fits fine. You can find them used at reasonable prices, B&H had one a while ago.
Another option is the Zhou case for Canon P - it fits great and gives a nice handle to the camera.
Hope that helps.