Different materials should not be an issue at all, in fact almost all of my thin-stacked cuts are some form of stainless or exotic material, and my sacrificial plate is almost always AL.
If I can weld the stack together, I weld it, otherwise use a thicker bottom and top plates and screw or rivet together.
I prefer to stay between the 1 - 2 " total stack height, if needed I use the sacrificial plate as the thickness make-up.
Like Brian suggested, use lower flush pressure.
For the tech .... mehh.... just pick whatever you think is best.
In this case pick something for the stainless material, the AL will get cut just fine.
You will not get the optimum speed anyway, but it likely don't matter when the end result is a few hundred pieces/cut.
If your washers are small, I wish you Good Luck finding them in the tank!!!