I think that I found the answer but want to confirm:

1. One can run a dura ace 7800 or 7900 crank and chainring with 9000 everywhere else, correct?
2. Can the 9000 chainrings be installed on the 7800/7900 crank?

From my understanding, the chainring spacing on the 9000 is slightly closer, on the order of a few tenths of a mm.

Thanks.