I got married in Arizona in October, but it was still in the high 80's/low 90's. I actually wore a hoopskirt as my first layer, and it was wonderful! Hoopskirts keep all those layers of tulle and crinoline away from your legs, and lets air in. I didn't feel hot at all. It was also easier for dancing - DH and I do ballroom dancing, and I wouldn't have been able to move as well if I had just worn a petticoat. (I actually did wear a petticoat over the hoopskirt, b/c my dress was very full, but it worked well and looked great.)
There are online sites where you can buy hoopskirts; I'll try to find the name of where I bought mine. It was adjustable, which worked well, and they also have different sizes (ie, 1 hoop, 2 hoops, 3 hoops, etc) depending on how wide you need it to be. I highly recommend using a hoopskirt!
Oh, and as for the bra, I definitely needed one, and I wasn't able to sew cups into my dress (wouldn't have worked), so I just bought a strapless "merrywidow" type bra from Victoria's Secret. I loved it, and it also wasn't hot. Quite comfortable, actually.