You ask a few different questions.
- Your gauge will probably change when knitting flat compared to circular. So make the hat first, then swatch flat to see if the same size needles will make the scarf look the same.
Adjust needle size and # of sts if needed.
- It's my experience that k1,p1 ribbing is more stretchy than regular stockinette. Remember that as you try to decide how many sts you need for the hat. Also, k1,p1 rib can be tricky to keep in pattern as you decrease. If you can match gauge to use your pattern that would be helpful.
I recently made several different hats and was surprised how stretchy the ribbed one was.
The skinny one on the bottom right and the light grey one with the stripes are made out of identical yarn. The skinny one is k1,p1 rib, compared to stockinette for the striped one. Dh tested them for fit.