Re: using yarn that was not a part of the pattern:
If the pattern used dk weight (aka sport weight) yarn...and you changed up to worsted weight yarn...but the same formula...it would make the hat gigantic.
It sound to me like you have a pretty good grip on your pattern!
I agree with Jan: I prefer to use two circular needles for the top part of the hat when the stitches are reduced to such a small number that they will no longer fit onto one circ!
Here is a good YOUTUBE video about using two circs:
If you can suffer through Cat's illustrations and her (to some people) irritating voice, you will learn a lot, and never forget how to use two circs in the round for tiny spaces!