It's not against union rules. The 9th show is paid at double-time, unless the previous/following week has only 7 performances (remembering that the Broadway "week" is Monday-Sunday), in which case it's just paid at the regular rate or an additional 1/8 of salary.
And yes, in most cases everyone in the company DOES do all 9 shows. We don't like it, but it happens.
"What- and quit show business?" - the guy shoveling elephant shit at the circus.