Pre-washing is a great idea...that way you will see how your non-cottons behave and if there is a problem with colours running. If the weights are similar, there is no problem combining, though some non-cottons are slippery and a little more difficult to stitch. The important thing is to get started and you will learn as you go. Don't be discouraged and if you get stuck....ask away. You will always find a solution here. Happy quilting.