Scrappy quilts are so much fun.. You never know until you finish just what you'll have. Don't be afraid.. Just sew and enjoy the "no thought" time.. The only rule I used was I didn't use a piece that was like the last piece, and I didn't use a piece that was so close is color or scale that you couldn't tell there were two pieces of fabric..
Besides that, it's your quilt.. If you pull a piece you really don't like, toss it back.. and say, oops.. blew that one! then grab again! But don't worry about clashes, they look great in the end and give a quilt some life and joy!