Your spoke lengths are correct assuming 32 spoke wheel build.TACSTS said:
If you use 296mm spokes on the non-drive side of the rear wheel the spoke may stick through the nipple a very small amount into the slotted head on the nipple. The difference is so small though I wouldn't worry about it.
I believe there is no advantage of using 16 mm nipples over 12 mm nipples unless you need them to project farther out of a rim with a thicker spoke bed. I believe that the actual threaded length inside all nipple sizes is the same, just the wrench flats are out further. Maybe someone else can comment on this?