If you're not yet a certified diver, then you're far ahead of yourself. You shouldn't even look at a speargun until you have 25 Gulf dives under your belt, and probably shouldn't consider purchasing one until you have 50. Experienced scuba divers kill themselves hunting fish and lobster. Two guys died in our area last year while spearfishing. Spearfishing is very easy, but breathing Gulf water is very hard. Learn to manage yourself underwater on scuba, and then add things like cameras, spearguns, lobster snares, etc.
I suspect you won't follow my advice though, so I'll say a 48" Biller is the gun I would recommend. Certainly don't go any shorter.
I don't know anything about Sporasub, but I wouldn't go any shorter than 90cm on a railgun (I shoot a 100cm Mako). Railguns are harder to load and use than a wood gun with a butt. They also take more finesse, as you're shooting a thinner shaft with less mass. If you miss the stone shot, you have to be gentle or you'll have a bent shaft by the time you string the fish. Don't forget you'll need a dive flag too.
For the love of Neptune, don't shoot "any fish" you "really see". Aside from what's legal, don't shoot a fish you're not going to eat. Most divers like to look at fish they're not eating, and that's harder to do if folks are murdering every fish they see

Alabama and Florida have wildly different spearfishing laws, and certain fish are off limits to spearfishers in both states. It's up to you to know these laws and follow them properly. Don't shoot a fish unless you know that it's legal before you pull the trigger. Remember that water makes everything look bigger, so a fish that looks "just" legal, is too short.