My philosophy is "you put them together they go together". I have made quite a few quilts. It does look odd when you first begin a scrap project because you only see a few fabrics then. But, as you make more blocks and it gets bigger, that's when the magic happens. My only two rules for scrap quilting is that I won't repeat a fabric in a block (if I can help it) and I won't put a fabric piece next to itself. Other than that, your options are open.