Hi, welcome to the forum. I'm not an expert but I have asked the same questions before and thereafter tried it out my self.
So from experience, the answer to your first question is yes. It is safe to apply optiseal after GPS. However, note that the polymers in the sealant part of GPS and optiseal itself will continue cross linking for a few hours after application so it's advisable not to wash your car after application for a day or so. Not that you can't, but for best results, try not to.
As for your second question, I do layer Optiseal with a layer of OCW. The advantages of doing so is that it gives the car a slick feel and a depth in the shine. I have added two layers of OCW after optiseal before and the shine has a lot more depth. I was told that it would also extend the durability of Optiseal some what. However, as I reapply Optiseal every two weeks after a wash (not necessary but I do like doing this), I can't really comment if it's true.
Whatever you decide, try not to touch or stroke the finish after application of either optiseal or OCW. It's very tempting to do so but I've found that leaving it to sit for a few hours seems to give a more even shine without any smudges or dull spots.