I had the opposite problem with my first wheel build. The spoke lengths were ok for a 12mm nipple, but too long for a 14mm nipple. I initially laced with 14mm Pillar nipples and bottomed them all out before reaching proper tension. Tore the wheels down and used 12mm Sapim nipples and that did the trick. As mentioned above, you should be able to go with longer nipples to "fix" it.

14mm Pillar brass nipple on the top.
12mm Sapim brass nipple on the bottom.

2 spokes of the same length.

Bob