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a few answers hopefully!
1. They are there to allow you to stack filters, so you can have up to 3 - perhaps a warm-up filter, an ND, and a grad to give you a nice warm long exposure with a balanced sky for example. It does give you that little bit more flexibility! I believe its best to use the inside filter where possible, to reduce light leakage and reflections.
2. It does depend what you want to or expect to take pictures of. If you want to take longer exposures or avoid blowing out bright subjects, an ND will help, and the stronger they are, the longer the exposure. I'm guessing if you've got the grads you're happy you know how you want to use them? The reason these 3 filters are recommended is because they solve problems and allow effects that you can't so easily correct for in Photoshop.
3. My personal opinion is that I wouldn't limit yourself to P-Series polarisers, as I tend to find it is one filter that stays on my camera most! Probably best to get a decent screw-in polariser with a front thread, which you can then add the P-Series adapter / holder on to.
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