if you want to increase (or decrease) the amount of seam allowance on your pieces, it's hard to figure it out from most written instructions. the most fool-proof way to get accurate results would be to draw each finished piece on graph paper and add your desired seam allowance to that. even then, i wouldn't trust it. also, smaller stitches will keep fabric from fraying better than regular size stitches. so if there's any doubt, go down a few increments in stitch size. and grain does matter regardless of fabric content.
and i love the zipper idea. i think nancy's notions has the poly zippers by the inch. is poly strong enough or does it need metal? i never saw separating zippers by the inch. tell us if you find them.