And it's not implausible that Starfleet might encompass multiple service branches. The Canadian Forces encompasses the Canadian Army, the Royal Canadian Navy, and the Royal Canadian Air Force, all under a single service force.
Kirk did say Starfleet was a "combined service" in "Tomorrow is Yesterday".
Mr. Laser Beam wrote:
They'll just have different ranks, like Major, Colonel, General, etc.
It's not like they even need to have army-style ranks, that's just a matter of tradition (Earth tradition, and it's not even completely universal here) and it's pretty much only cosmetic. Every army rank has it's naval counterpart.