Sounds like you chose Celebrity for the RIGHT reasons.
Regardless of what you may read by many posters, X is a more upscale, dressier cruise line. There will always be some like 'Jean' and 'luvtocruise99' that try to fight the system. They feel that since they are paying for a vacation they don't have to follow any rules, or they LOOK for any ambiguities in the codes so they can defy them.
You WILL NOT be out of place in a gown on formal nights, You will be in the majority.
I never feel out of place in a tux, or dinner jacket....
Have a wonderful trip.