Been playing around with this today - 8 strands of various weight yarns on 20mm needles
It was a little awkward at first, especially for someone who knits Continental instead of English, but I got the hang of it, despite having two different brands of needles.
Got a surprise just now, having received a bunch of needles from a friend who doesn't want them anymore, and yahoo -- another 20mm needle of the pointy variety which is much better for knitting in this size.. hmmm i wonder if i can get Addi Turbos in 25 mm circular? That would be one quick sweater!