oh wow...Ginger, you don't have to worry, Mexico is not like Cuba where Americans can't go (even..UN just voted today to end the embargo), therefore you will look very hot even with a bit of seasonal hohoho fat on you. My mom's confidence went up 100% after our mom-daugther trip (she called it sisters trip) in Playa Del Carmen.
I am thinking of restart my hot yoga routine after the new year (due to the lack of motivation, I postpone it til new year...heehee) to tone up a bit...and hire a dancing instructor...so I can have a perfect first dance with my FI.
As for the taffeta, people can add extra $$$ to make their dress with silk taffeta, it is very light, soft fabric with a subtle shine. However, it isn't suitable for TTD. I did my TTD style engagement pics in a runway silk chiffon Prada dress, and errr...I totally wrote off the dress after the session.