I have an FSA Gossamer compact that I'd like to put on my rain bike (which is Shimano 9-speed). That work? And vice versa--can I put an old Ultegra 9-speed crank on a 10-speed drive system? Thanks!

There are some that will say nay but I've used 9-speed cranks on 10-speed systems and vice-versa.

Theoretically the chainrings are a little closer together on 10-speed.

I did see someone once use an OLD-school 42/52 crankset from the 6-spd days with a 9-speed chain, and the chain dropped betwene the rings.

I have an FSA Gossamer compact that I'd like to put on my rain bike (which is Shimano 9-speed). That work? And vice versa--can I put an old Ultegra 9-speed crank on a 10-speed drive system? Thanks!

9 and 10 are pretty much interchangeable. The only people who don't thinks so seem to be the manufacturers. It seems that they may have some sort of incentive driving their beliefs :)