So the question is if the thread will make the block shrink if you use cotton on one side and a blend on the other, right?
It's not like you are going to wash a size 10 cotton T that is made in Sri-Lanka on the hot cycle in your machine and it comes out just large enought to fit a Barbie. I think the shrinkage factor on thread is so minute that it wouldn't even be a blip on the quilt police. Besides, there will be quilting stitches to secure the whole top. Me thinks you worry too much.
Beautifully resolved btw!