Gracie:
I am a full bronze dancer and have been to 5 or 6 comps. so I'm no expert, but maybe I can help. I say just enjoy. It's natural to be nervous and everyone is - even the most experienced dancers.
Why do you dance? If like me, you love it. Use that on the dance floor. The judges like to see people who are having a good time. Yes, the dancing is judged, but no judge sees every little bobble. And your instructor, if he is worth his weight, will help you and lead you through any mistakes. It's really all about enjoying yourself and getting experience at your first comp. (or any comp.) The more you go to comps the more relaxed you will be - it's always scary when we do things for the first time.
As far as costumes, it sounds as if you have planned well. Your instructor will not let you go to comp unprepared. He knows what the dancers wear and you should show him what you plan to take and wear. Usually, the dance heats go pretty fast and there's not alot of time to change latin dresses between the different dances. I usually wear the same latin dress from start to finish if all dances are same day without many breaks.
I know most people don't think the wc swing is in the same catagory as maybe the chacha, but I have seen it danced to be very sexy and playful. I'm sure your choice of latin dress is good for all latin dances. Again - I think your instructor will know best.
I'm going to a comp next weekend, with a new instructor, and I'm also alittle shakey - but comps are what I like to do best. I love to dance, but I love to watch the other dancers too. I get great ideas. I discuss with my instructor and friends. Everyone there is very supportive. No one there wants to see other dancers have a bad dance. (At least at our levels.) I'm sure you will have a wonderful time and can't stop talking about it for days- As long as you go with the mind to enjoy every minute.
Good Luck!