I think you're mis-applying the logic relating to nipple thread length. It would ONLY matter if the spokes extend beyond the tops of the nipples. Since you're describing a situation where the spokes are too short rather than too long, there's no issue. In fact, for your situation, you'd want more, not less thread length. So, feel free to buy any14mm nipples as long as your spokes don't reach beyond the nipple heads currently.
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