Re: Starfleet is the U.S. Coast Guard
I think as a "combined service" Starfleet would draw whatever it thought was good from those services into its own practices.
IMO, the OP's notion of new pre-officers touring before going to an Academy and getting to choose what they want to specialize in not only fits with the Starfleet notion of the ranks of crewman and yeoman, but is also just plain cool.
Does that make Starfleet the Coast Guard? Nah. "Combined service."
"If you can't take a little bloody nose, maybe you ought to go back home and crawl under your bed. It's not safe out here. It's wondrous, with treasures to satiate desires both subtle and gross; but it's not for the timid." - Q