I would think you should be fine modifying the pattern as long as you remember to do that to the front and back. As for the increases, you take the number of stitches you need to increase, add 1 and divide into the total number in your row (ex: you need to increase 4 stitches evenly across a row of 30 stitches: 4 + 1 = 5, 30 divided by 5 = 6. You would increase every sixth stitch.) As for the increase, you could knit into the front and back of the stitch.
"Pinup Queen" by Annie Modesitt from Stitch N Bitch