Another long post I'm afraid...
Depends. I dislike overemphasising military stuff in Trek but that doesn't mean that there can't be any combat ever (TWOK is obviously great even though there's quite a bit of fighting, but I really didn't care for the Dominion war or Xindi war stories for example.) My main point is that I prefer Starfleet to be mainly an exploration force with some military duties. Marines' whole point is that they're soldiers first and foremost, so they don't fit in that picture.
I somewhat agree with "soldiers first and foremost" beign problematic (and the USMC are some of the most first and foremost" soldierly types around). Which is why I favour them being restricted to a specialist unit rather than a top-level public facing command (ala the USMC), in fact the way I see it, even the term
Starfleet Marine Corps signals they are a lower level unit (equivalent to called it the US Navy Marine Corps), ala the Royal Marines (which are infantry trained members of the Naval Service rather than a seperate organisation), rather than the
Federation Marine Corps (a top-level (four to five-star) command, which would in my 'head-canon' be the commando elements of the Federation Reserves (equivalent of the US National Guard or state forces).
Sure, but 'Starfleet Tactical' is probably just a department of Starfleet that devises tactics. It's like those guys in Pentagon who devise plans in case Canada attacks and whatnot. It has nothing to do with any Marines (which probably do not exist.)
Those specific redshirts certainly were. But it doesn't follow that if ground forces were required they would be from different organisation or branch. We saw Starfleet ground installations manned by similar personnel.
Why? We certainly have seen Starfleet personnel fighting on ground, doesn't mean they're part of some unique division. Besides, unlike DS9s highly implausible depictions of 24th century ground warfare might lead one to believe, given the technology large scale ground combat would be highly unlikely. Establishing space and air superiority would get 99% of the job done in most cases. Besides, even in DS9 those ground forces seemed to be regular SF personnel.
Hypothetically, Starfleet Tactical might be arranged similarly to this:
Tactical Advisor to the Chief of Operations - Day-to-Day Commander of the Branch, likely a junior flag officer during the early years, probably a "four pip" rank flag or general officer.
-- Head of Analysis - equivalent to DARPA, ATEC and similar in-service military units. Shelby headed the Borg Analysis Team here. Liases with Office of Science Ops for lab work. Probably a "three pip" rank flag or general officer.
-- Head of Special Operations - Contributes tactical expertise and manpower to situations to complex for generalist starship personnel. Probably a "three pip" rank flag or general officer.
---- I (Fleet Protection) Corps - specialist "base defense" units ala the RAF Regiment, mostly Starfleet Security personnel, but augmented with Science and Engineering experts, the "troops" at AR-558 would be primarily these. Administratively lead by a junior flag or general officer (one or two "pip").
--- II (Desert Recon) Corps - hot-weather rescue/assault and all-weather recon, supplements SI's Ranger Teams as needed in the later, aka as the "Sehlats" to the displeasure of the traditional Vulcan recruits. Administratively lead by a junior flag or general officer, but field leadership is at senior officer level (Captain/Colonel to Lt Cmdr/Major)
--- III (Polar Commando) Corps - cold-weather and mountain rescue/assault, aka "Starfleet Marine Corps", mostly Humans and Andorians traditionally, potentially Caitians. Administratively lead by a junior flag or general officer, but field leadership is at senior officer level (Captain/Colonel to Lt Cmdr/Major)