If you really want your questions answered go to
http://www.spearboard.com. As far as putting a reel on a gun I would advice against it for scuba. That setup is primarily for freedivers. I hunt with a 54" SS commercial gun and for Florida hunting they are about the best. Mine is rigged with a freeshaft and a hybrid lineshaft. In my opinion freeshafting is the only way to go, it makes you pick your shot instead of just shooting randomly. I now get a higher percentage on stone shots.
http://www.slinginsteel.com is where you can read up the guns. Kevin Bruington makes one awesome gun and stands behind them 100%. You can contact him on spearboard or at slinginsteel and he will answer any question you have.
As far as carrying fish, I, along with the majority of your shooters in the state carry their stringers close to our body. Sharks are opportunity feeders, they do not want to get hurt getting a meal. If they do get curious just poke them in the nose and they'll go away. Every so often you get a brave shark that you might have to be more aggresive with, i.e. charge him or put a shaft in his gills. Don't get me wrong I don't kill sharks for the sake of killing. Just self defense.
As for the powerheads, yes I carry a .223 with for self defense. It is legal to hunt with in Federal waters. I wouldn't recommend carying one until your skills underwater are honed to perfection because it adds 1 more dangerous variable to the equation.
Keep in mind you are in Florida so advice from inland and West Coast USA hunters will be good advise, think about how you will be hunting and where. Especially on gun setup. What works in the kelp beds off CA freediving might not necessarily be the best choice for scuba on our reefs.
I know this post will probably start a storm but keep in mind, this is what works for me. I hunt both coasts of Florida. Scuba spearing can be tricky since you have the noise of the regulator exhaust, so don't get discouraged if in the beginning you scare more fish off than you shoot.
Good luck, be safe and above all have fun!