You can't always trust the manufacturers or even Sheldon on some of this stuff... there are just too many variables.

I had a 9-speed chain that never worked right with my 8-speed compact crank - it'd skate or get caught between the rings - no amount of futzing with it helped... gave up and bought a new 8-speed chain and it worked fine again.

I have a 10 speed cassette (ultegra), shifters (DuraAce) and chain (shimano) that work just fine with my 9-speed Dura Ace crank, FD, and RD... Shimano says I need a 10-speed crank but I've had zero issues with front shifting.

But narrower chain on the rear is almost never a problem.