I would use the 18's and make sure my stitch length is the same as the original, so I am not punching new holes. New holes would weaken the leather along the stitching line.

Is the stitch length critical if I use the leather needle? I'm fairly sure I can't match the original holes exactly. Even on my longest stitch, they aren't the same length.

I can get new needles if that would be better.

Leather is not like fabric. Fabric has fibers that are woven or knitted together to make the material. The needle pretty much slides between fibers rather than cutting them. Leather is an animal hide. Once you make a hole, it's there forever. If you make multiple holes in a seam line, you've created a a line that will tear easily and conceivably act like a scissor to cut the hide. Leather needles are designed to be used on leather and minimize this problem but will not eliminate the possibility of the hide tearing if you sew over the same seam several times. Ideally, carefully sewing with the same size needle in the same holes will help minimize this problem.

Can you hand-sew the zipper in, using the existing holes?