For the final decrease round, k2tog all around is a good idea because it will take you down to 14 sts as Sue mentioned. That's a reasonable number of sts to fasten off (and if it seems like too many, immediately do another k2tog round which takes you down to 7sts).
A gauge swatch is a good idea since you're changing yarn. That way you won't waste time knitting a hat that's the wrong size. Even in the stratchy k2p2 rib, it'll still be a help to you.