Got ahold of my former partner. His SLUG of choice was the same as mine for my MOSS 590, Remington Solid Copper Hollow Point Sabots.
His best Shot Load was also Remington Double Ought Buck Premium loads. I used something else for my shot load (Fletchets). He said his gun ran smooth with that diet, I have no reason to think they wouldn't. He also said to fire only high powered loads as light loads tended to jam it up.
I didn't think that mattered too much since you can switch the gun to pump action mode... but what do I know? I don't own a SPAZ 12.