I'm assuming you're going to knit flat so you can do intarsia right?
If you want to do a 1x1 (k1,p1) rib you need a multiple of 2, if you want to do 2x2 rib you need a multiple of 4. I hate seaming so I always do them in the round so I don't know exactly how many extra to cast on for seaming..maybe 1 or 2.
If you want to do it in the round you can always add the design with duplicate stitch.
To make a double reversible hat you can either double knit (fairly difficult with decreases so it's not really for a beginner), or you can do it this way.