Sounds to me like you're really over-thinking the process here. You've divided your photos into piles, but you don't specify what those piles are. People? Vacations? Holidays? If you have photos that you don't have a pile for, it sounds like you need to add another pile. LOL
As far as do you have to use all the pix? NO. They're your pictures and it's your scrapbook. YOU make the rules. If you don't like the picture, don't put it in the scrapbook. If you REALLY don't like the photo, throw it away. Yes, I really said that.
If you like it but not enough to scrapbook it, put it in an acid-free photo storage box. They're $3 or $4 at Hobby Lobby or you can buy them at Michaels with a coupon. If you can't find a pile to put the photo in when you're sorting, maybe you should think about do you really want it anyway, and if not store it in the box. Store doubles in the boxes too.
When I started scrapbooking 11 years ago, I thought I needed to put every single photo, good or bad, in my scrapbook and I needed to scrapbook chronologically. I really put a lot of pressure on myself and that was stupid. Cause if you do that, you're going to die someday and there will be a million photos stashed somewhere in your house that aren't in the scrapbooks. Now I pick out my favorite photos, the ones I really love, the ones I want to tell a story about, the ones I want my daughter to know about, the ones that are meaningful to me, and I only scrapbook those. The rest go in photo boxes. And WOW do I feel better!
Recently I also started NOT going in chronological order. Scrapbooking is so much more fun now. Because frankly, by the time I get to THIS year in my scrapbooks, I'm not going to remember what happened in the photo from this year, unless I journal very extensively about it, and if I'm going to do the journaling I might as well just go ahead and make the whole page. So I alternate. One day I do current photos and the next day I use older photos for my scrapbooks.
Just do whatever YOU want. You're the boss!