Hi, and welcome. A couple thoughts:
Unless you want to limit yourself to time trials, you're going to have to get comfortable and proficient riding in a group. As FF said, find the local fast group ride, and do that ride. You can start at the back, and as you get used to finding the draft, you can move up.
An 80s steel frame bike isn't going to hold you back, especially in Florida. But DT shifters will put you at a disadvantage. It's not disqualifying, but it's an extra level of complication.
You absolutely can use your commute as training. I have for years.
Re cyclocross: Actually a 70-80s road bike can be adapted to CX. I did my first season of cross on a Trek 660 with a set of knobby tires. It worked great in dry September, but by October it got bogged down in the mud.