Handquilting thread also has a wax like additive, to make it go smoothly through the fabric and keeps it from knotting up also :wink:
If you choose to mix your fabrics, be they new, old clothing, polycotton blends etc... just be sure that you prewash them. That is one of the biggest issues with mixing fabrics...they all will shrink differently.
Polycotton blends also don't crease well when pressing seams in your blocks. Not saying this is bad, but just something to remember while piecing and measuring for accuracy.
It all boils down to this...it is YOUR quilt and you can use whatever you like :D:D:D