Can the spiro bean be reflected off a third non-spinning mirror and still hold the pattern? Or will the pattern be to big? The third mirror will be around 1 inch.
Doesn't matter as long as you can focus the laser to the distance you're gonna use the spiro. But as things said, the closer the better. I'd leave 3 or 4mm just to eliminate vibration issues.
As long as the entire spiro "shape" remains on the surface of the final mirror you will be fine. Remember to account for both the size of the shape and the limits of the movement of the final mirror.