Mohair and boucle won't lend itself to lacey stitches unless you use really really really large needles. And then you need to keep it to a basic yo k2tog stitch. The 3mm will probably be too small to knit the hat on. The fuzz from the mohair fills in the sts so you knit that type of yarn on larger needles than you'd think. Take a look at the gauge on the yarn label and see what size needles are on it. Then maybe go up a size or two. But once you figure the right size needles a hat and scarf combo sounds like a good idea. Boucle yarn needs a larger needle too, and plainer sts work better with it as the texture comes from the yarn. But a basketweave st is good - k6 p6 for 6 rows then p6 k6 for 6 rows.
sue- knitting heretic