Well when I am testing Mac II, SE/30, I dont need any of the negative rails. 

I need the 5V, and the 12V. Thats when I get sound and everything. But youll loose communication, and I am unsure of floppy drive requirements. 

The machine wont run without 12V, and it definitely wont run without 5V. The 12V i assume is part of the reset management circuitry, which explains why it needs it. And its for sound as well.