I've mixed materials before. I love to add shiny bridal satin left over from the kids wedding dresses. I've added polyester fabric from left over children's dresses. Ive yet to make a t-shirt quilt...some are cotton/poly...but I will some day. And i love to embroider so I make new fabric from time to time, and soon, I plan to acrylic paint some fabric...so on and on I go....No rules.