Since Ingrid has obviously not seen this yet... I feel safe responding to part of your question: Yes, using 2 strands changes the gauge! I do a lot of 2 strand knitting and, because you are using "thicker" yarn and bigger needles, your guage changes. Yes, the amount of yarn changes with this but it depends on your gague. My advice? Do a swatch to see exactly what the gague will end up being.
As far as needle size - I've never used this particular yarn so I'm not one to advise. I believe that you have a bit of flexability in so far as how tight or loose you want the knitting to be. Someone else will be better able to guide you here.
Good luck and keep us posted!
PS - not dumb questions as all! Don't ever worry about that here - everyone's pretty awesome about answering whatever questions get tossed out! Especially Ingrid!