combining fabrics is perfectly ok as long as they have similar (care needs) ie: washable/dryable together-
many many people use poly blends all the time and combining them with cottons is always a good choice. about the only difference you will find apparent is the poly blend will not be as soft as the cotton with future washings- they tend to keep their....feel....where cotton just gets softer and softer.
mixing textiles is wonderful in quilts most of the time :) have fun constructing your first large quilt :)