My question is would they be specialists within Starfleet Security (eg Bob Fletcher's "Starfleet Special Forces") or would they be part of the "Expeditionary Force"?
I wasn't particularly familiar with that unit, but after reading a bit it seems like... alittle bit of both? In my version here, Starfleet could be considered somewhat analogous to the Coast Guard. I think, in the context of my reboot, it makes more sense for the Expeditionary Force to be handling most of this, augmented by Starfleet Security.
My Starfleet really shouldn't have much in the way of actual "ground forces"... it was the entire point of the Expeditionary Force to provide that. Starfleet has Security, and within that there could be some that are a bit better trained and equipped who could do the job adequately, but having an Expeditionary Force unit would be ideal.
ALSO keep in mind a third option that we haven't talked about too much here. In my universe, some of this duty could also fall to member world forces, especially if it's a more long-term mission.
There's a progression here with Starfleet... it was never really supposed to be the main fleet that conducts space warfare. It becomes that, because of how huge and monolithic the organization becomes, but it was never intended. Starfleet very much was supposed to be something more akin to a research organization combined with a coast guard. Member worlds fleets were supposed to handle the lions share of warfare situations.
As the Federation matures, there's a shift in the general culture and attitudes away from staunch independence and sovereignty of most members to identifying more as the whole... the Federation becomes more and more... federal as time goes on, with the side effect of Starfleet seeing ever growing contributions from member worlds.
By the time we hit the TNG+ Era, most member worlds no longer field huge defense forces of their own, preferring to rely more and more on Starfleet... despite such duties officially being outside of its mission profile. The Expeditionary Force, by proxy, also grows as member worlds depend on the Federation more for defense, rather than their own forces.
The division color makes sense, but I actually see the Marines using more variants of the pajamas depending on the mission profile. The black/green shoulder would be the day to day work uniform aboard ship or on base, but if in the field a camouflaged variant would be employed.
Oh yeah there would be a few options, especially for combat gear.
I'm actually coming around a bit more on my reboot universe as a whole to not be quite as slavish to Prime canon. I can do whatever I want here, by definition. If I think something works better... there's no reason not to change it.
Uniforms were something that came to mind. Trek changes uniforms alot. There were obviously practical reasons for that out of universe. In-universe... we might drop a few iterations and change some things around.
I was specifically thinking of the Movie-Era uniforms and the talks were having about dress uniforms and not. I'm going to alter that in my universe, albeit fairly mildly. The "Monster Maroons" exist, because they look great, but i'm going to slightly edit their use. Not a great extent, but mildly. An important note, I don't think uniform codes of a multi-national organization 250 years in the future need to rely on exactly 21st century military uniform regulations... so... it works a bit different but we will use some modern nomenclature for ease of discussion.
Starfleet begins to phase in the "Movie-Era" uniforms in 2270 (the Motion Picture uniforms don't happen... at least, by and large. Some variants could exist.), although the Expeditionary Force had already been using the jacket-version since 2245. Given the general turbulence of the 2250's/60's, Starfleet began to tighten up and demand a higher level of formality than had previously been the norm in the fleet. A Starfleet division-colored version of the Marine uniform was adopted as a "service uniform" for Starfleet officers. Starfleet wanted the bridge of a starship to project a level of formality, given their missions often involved diplomatic roles or otherwise needing to represent the Federation. As such, officers serving on the bridge (and select other postings, stations, etc.) were expected to wear the service uniform while on duty. Other, less "on display" stations could choose to wear the service uniform or a more relaxed "Class B" uniform so to speak, which is essentially the "Star Trek Beyond" version of the TOS uniforms. (The service uniform would persist in Starfleet through the early 24th century, with the Class B getting a fairly radical redesign into the early Season 1/2 TNG uniforms, before dropping the service uniform and moving to the S3 TNG and DS9/VOY type)
The Expeditionary Force had already been using the service uniform for more formal functions, having moved to adopt a variant of the "new version" of the Starfleet uniforms that also began service in 2245. (The EF was basically wearing the SNW-style uniforms as duty uniforms, but the a male version of the half-zip variant.) The EF is a bit more steeped in tradition, so when Starfleet eventually moves to the TNG-style "dress" uniforms and settles on essentially a single uniform without really having a real class a or class b, the EF still does retain a version which looks much closer to the Never Actually Used TNG-style uniforms they planned for Generations.