Betty Jo, go and mix at your heart's content! It's such a lot of fun! There is no reason at all why you should not use different types of fabric together. It actually adds an extra dimension to your piece. All fabrics reflect light differently, so you will get a subtle contrast when using silks with cottons, or linens, or wool, or (quilt police turn away, please) synthetics, or if you overlay a fabric with a sheer, like organza.
I used a silk jacquard for the body of a dress and a standard 100% cotton for the bodice. This dress has been washed very carefully by hand and then drip-dried on a hangar. See how it looks:
The only limitation is the way the piece will get washed or cleaned, so testing all fabrics is always a good idea.