I just meant 'flatten' it slightly so you can get a good measurement. Take the needles out if you want, but it's harder to be accurate when it's still on the circs and might be a little lumpy.
When you work in the round you don't turn to knit on the other side, so to get garter stitch you have to alternate a purl round with a knit round. Stockinette stitch in the round is just knitting all the rounds because you always work on the RS.
Yes, 'twin pins' is what you call circular needles over there, and the length is measured tip to tip, not just the cord. You can work a hat on 40cm needles, but would still have to use dpns or longer ones (in magic loop) for the top part when you decrease stitches. You can use a longer one all the way through using magic loop or single loop, and needles from 60 to 90 cm long.
Finally - yeah, just cast on for the hat and work a few inches, then measure your gauge. It'll be a lot easier to 'flatten' it out with a larger piece.