Just knit back and forth for a couple rows on your 16" and then join into a round. You can't do Magic Loop with that small a needle, but you don't need too. When the circle gets too tight at the top, due to decreases, you would need to change to DPN's because you can't stretch those few stitches around the cable... No worries,, take another circular .. closest in size as you have, and put half the stitches on each needle and countinue in circle.... OR just take a needle and thread through the remaining stitches and put up tight when you can no longer decrease...... You can use the little bit of cast-on thread leftoverto sew the few rows together at the beginning of the hat. Learnng something new is always a little frustrating, but well worth the efffort... you will end up loving them... if you hate seaming as much as I do.....