So one thing I always wondered about with audio mixing is that clipping should be random? Like if the sine waves somehow all line up the momentary amplitude could be much greater than the typical amplitude.

And this would mean that there isn't a fixed volume above which you get clipping and below which you don't get clipping. Playing tricks with phases could prevent/cause clipping.

Is -20 db then simply a rule of thumb for preventing occasional clipping?

You're talking about the difference between Average amplitude and Peak amplitude. Yes, the peaks will be higher than the average, but not by a constant amount unless you're working with a pure constant sine wave. It will depend on the sound you're working with. The phase is part of that sound so generally you can't mess with it.

-20 db would just be a starting point, you could adjust up or down from there.