I love to work with sport zippers and it's always difficult to find them the right color, never mind the right length. Standard zippers are lightweight and easy to just shorten from the top but it's trickier when working with a bulky separating zipper. I now buy them long and just shorten them to the length I need. So if I need an 18.75" long separator I can get exactly that.
Before shortening the zipper I go ahead and machine baste it into the opening beginning the stitching not at the raw edge but at the seam intersection. This will give me room to turn the excess tape into the seam.
For plastic zippers I use a diagonal cutter (the Fiskars one is my favorite because it has a fine head) and just clip off the teeth.
For metal zippers I pull the teeth off the tape with a pair of pliers. Once you get the hang of it they come off pretty easily. Just give them a good pull.
Now the excess tape can be folded into the seam (and you can stitch without worrying that you are going to break a needle on the zipper teeth!). This is enough to keep from zipping off the slider. If you like, you can add a new top stop - they are available in brass and nickel from Atlanta Thread & Supply and are just pinched on with pliers. www.atlantathread.com