Hi D Vee,
Don't worry about the powdered milk - I routinely omit it with no ill
effects, along with the oil. I add a few more Tbsp of water to make up the
moisture from the oil.
The powdered milk is supposed to make a smooth shiny crust or something. I
don't know about adding powdered soy milk; I've never tried.
Happy Baking!
Jess
------------------------------