I know folks are concerned about the weather - mind if I share a little wisdom I've picked up from living in west-central FL for 20+ years?
1. Yes, the forecast highs are HIGH, and 90* is darned warm, even for Floridians. But the high is not going to be our race temp! The low temp the night before is likely to be your starting temp, and temps aren't rising as quickly as they do in summer, so expect maybe a 10-12 degree rise from start to around 10am.
2. The sun is pretty intense down here. That can make your skin feel hot, even when the air temp isn't bad. Pouring water over your arms/legs/shoulders/head can help quite a bit. And don't forget to use sunscreen! I saw some gnarly burns late in the half last year.
3. Yes, you need to hydrate. But don't over-hydrate! Too much water is as bad - often worse - than too little. Google "hyponatremia" and be careful. Use your product of choice to replace electrolytes lost to heavy sweating.
4. Back to those daily high temps... while they don't directly impact race conditions, they can if you're not careful about sun exposure, hydration, and/or overheating between races! Do as the Floridians do and take time to cool down in the AC, sip fluids all day, keep up with applying/reapplying sunscreen. Because starting your race with a bad sunburn and dehydration is no fun.
5. About costumes... go for cool and comfortable above all else! If you can make a race costume out of running apparel you'd normally wear in warm weather, you're golden. If your costume is more appropriate for temps in the 40s, I wouldn't.
6. I'm often asked "how" I manage to run throughout our brutal summers, when the heat index is over 100 nearly every run, and it's a very simple answer: I slow down. Really, more than any other things you can do to handle the heat, slowing your pace I think works best.