There are too many variables in this question to give a very specific answer (or even to refer you to a source that would be especially helpful). You have to think of several factors when you mix yarns: How they will look (this will depend, too, on the weave structure you are using), how they will feel (soft and supple for a scarf, firm and flat for a placemat, for example), and how they will finish (whether they shrink and full similarly if you plan a high degree of wet-finishing). And, when you are using different yarns even of the same fibers, there are differences in their behavior based on spin, ply, and thickness.
Usually, for a scarf, you’d be choosing a structure with a single warp and weft (plain weave, lace, or twill) for a light and supple fabric (or, if using wool for a warm scarf, for a plushy fulled fabric). For these weaves, you usually want the warp and weft to be the same fiber and thickness. You could also choose a supplementary weft structure, such as overshot, and place patterns in the border of a scarf (where you could use your bits of wool weft). But even these possibilities will depend on your yarns’ fibers, thickness, ply, and degree of loftiness. And you would have to be careful if your scarf’s ground cloth is cotton and the overshot patterning wool not to wet-finish the fabric to the point that the wool shrinks (since the cotton will not).
The upshot is: Try different combinations in samples!