12mm is normal
14mm is for machine built wheels generaly (guess they’re easier for machines to handle?), although CRC spokes come with 14mm nipples for an unkown reason. Some people say reduce the spoke length by 2mm if using 14mm nipples, others say the spoke should always fill screwdriver slot to reduce the strain on it and stop it cracking there, in which case nipple lengt would be irelavent.
For that reason I always use 12mm and whatever the DT calc says so I dont have to justify it one way or the other!
Alu nipples are bling, but almost always crack, so stick with brass. Some come with locking/anti sieze compound already applied, others need the spoke threads coating before assembly.
If the rims don’t have eyelets (e.g. Stans) it’s better to use nipple washers to protect the rim.
1-2mm won’t matter much, most shops only sell in 2mm incraments anyway, so either average the results or round down as spokes stretch about 1mm when you tension them.