Most of my weddings were shot with a single C330. I carried 35mm equipment for emergency backup, but never had to use it.
I used to mix 220 and 120.
Every once in a while I'd have to waste a shot or two (e.g. to ensure a fresh roll for the newlyweds' walk down the aisle) but generally it was enough to understand the timing of things and working within that.
You get quite good at unloading/loading film .