1. Three or four cross is really a function of hub flange diameter. Sometimes the spokes cross at the spoke holes on the flanges and make it hard to lace. Practically, I only use 4 cross for 40 spoke wheels.

2. If I were building the wheels, especially a "retro sort", I'd use 2/1.8/2 for everything. I don't see that you're gaining anything by using the skinnier spokes.