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Personally I would build the wheels with the correct sized spokes. Because, proper 😂
In case I was going touring and I only wanted to carry same size spares I would just take the shorter spokes. Good enough for emergency and you can repair your wheels properly, once you are back.
You will see spoke elbow failures every now and then. But I have seen very few snapped brass nipples.
On a branded brass nipple? Very unlikely.
Besides, on 12mm with a thread fully engaged, the spoke protrudes over the nipple.
On 14mm the spoke sits flush with the nipple end
On 16mm the spokes sits flush with the" mushroom" part of a nipple.As long as the spoke fills that portion, all is well.
With aluminium nips, it would have been an issue. With brass, not really.