Sounds like you are now enjoying the look of your random scrappy, which is good. Sometimes it has to "grow on you" as you construct it if it's (to use a cliche) outside your comfort zone.
Putting your plains in there helps tone down the busyness, I'm sure. As long as the plain colors also contrast with their neighbor, it shouldn't matter too much what fabric style the scraps are.
I'm making a beach quilt with (2.5 " wide x Random length ) strips leftover from each quilt I make. It's either gonna be ugly or fabulous, but it'll remind me of every quilt I made and person I made them for. Normally I like to pick a multicolored focus fabric and use scraps in those colors to go with it. Other times I just put on my crazy panties and run with the bulls!