no 14ga nipples will not fit 13ga or 12ga spokes, Measure it, don't guess. Also why do you want to replace the nipples? Are they twisted? Rounded? Corroded? If not I would just re-use them, be sure to lube them and the rim they are going on. I used to build my twenty inch bike with 12ga spokes in the rear and I would just re-use the nipples on that. I think I went through two or three rims on it but never a nipple (It's always bad to come down front wheel first on a 20' jump onto flat ground, always bad....) One thing that is hard to use is new, smaller spokes on hubs that have been drilled for larger spokes, usually it takes washers to keep the keep the spoke head from coming through the hub flange, so I probably wouldn't use new spokes.