If it is going to be washed after it is made, I would definitely advise prewashing the fabrics, otherwise go for it. There really is no reason not to mix different fibres - particularly in a decorative piece - look at all the antique embroideries around, they are mixtures of fibres, usually linenground & silk embroidery & sometimes wool as well.

I have developed a quilt design using a Dover "Peasant Designs" book that I am thinking of making the main background in dupioni silk. It was pretty thoroughly inspired by Tom Russell's TQS show, because I want to add lots of beads and possibly even sequins to this design. The thing is, it has appliqued birds and flowers on it and so I was wondering what you all think of using cotton appliques on dupioni silk, or should I get some dupioni silk pieces for the appliques while I'm at the quilt show in Philadelphia next week? I will probably end up hand washing it in cold water after I make it, but not sure about that either. Do any of you have experience using silk with cotton combination?