I'm in Miami now and before I got here I was dreading the idea of having to learn Spanish. It is true that wherever you go, people speak Spanish. In fact, no matter how white/asian/black you look they will still speak to you in Spanish first and switch to English (if they know it). However, in courtrooms they speak English, at firms they speak English, at school they speak English.... get the idea? You really don't need to know Spanish unless you are doing something immigration related, international, or public-interest where most of your clients will speak another language. Creole is another biggie down here, so if you know French that may be attractive to some employers but I really wouldn't worry about that part.
As for firms, White & Case, Greenburg, Ackerman, Carlton Fields, Hunton & Williams....
If you aren't at a Florida school be prepared to give a good explanation as to why you are interested in working in Miami (other than the weather). You won't be disadvantaged by starting your career in Miami unless you decide to move somewhere else later. You would be at a disadvantage wherever you go afterwards just because you don't have any roots in that area. That is why most people advise you to work where you plan on being at in the long term.