I hope this is fine as an issue, as this is not a bug but a question.

I am looking to play with default nameplates, but with the Larger Nameplates blizzard option enabled, but I would like to decrease the size of enemy minion nameplates. Neatplates can do this by reducing the scale of filtered units. Could you point me to what file this is implemented in? Also, I have never interacted with WoW API or Lua, but is it possible to modify the blizzard nameplates?

The main handling for scaling is done here, also here

But I take it you are simply trying to modify the default blizzard nameplates? In which case you might be more interested in the SetScale function that already exist for almost all frames. And possibly the NAME_PLATE_UNIT_ADDED event for when a unit nameplate gets displayed. Used here in NeatPlates.