I'm interested to see what you propose for the organization of the Lift Cavalry Troops.
I'd use the TOE given for the Lift Cavalry battalions of the 4518
th (Huscarles) in SMC and JTAS #9. While not a true Marine unit, I guess it's based more or less on Marine structure.
3. Royal Marines, Soviet Naval Infantry or USMC for Inspiration? (I prefer the Royal Marines they are better trained which reflects the traveller model in my opinion.)
To define the Marines, you should first define its roles and functions, and that is closely tied on your vision of the Imperium.
As I see it, the main Imperium political tool is the IN, and the Marines are its dirtside force projection branch. As such, they are under Naval command (as USMC is, if I'm not wrong).
Another important point is that they don't have extensive tail, using Navy for support. This is explicited about the Medical personnel in several places, and I guess it can be extended to th whole Marines (though they have their support branch).
From MT
![Stick out tongue :p :p](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
M, page 45, under Medical School:
Marines have no Medical Officers; they are treated by Navy doctors.
From MT
![Stick out tongue :p :p](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
M, page 48, under Combat Arm Selection:
Marine characters may select Infantry or Support
This also hints that Marine Cavalry or Artillery are very small arms, that can only be joined after a cross-training on them by former Infantry or SUpport memebers.
What are Marines roles and missions?
As said, they are the dirstside power projection branch of the IN. As such, their roles and missions are likely to be:
- IN base defense (along with IN members in shore duty, acting as Naval Infantry)
- Ship's troops, acting as Power Projection Forces, boarding parties and defense and internal seccurity inside ships. See that they also serve as gunners.
- Spearhead planetary assaults.
- Punitive raids (I guess mostly in anti-piracy missions and such).
- No doubt showing the flag and Shows of Force when needed, along with IN (after all, the ships are not seen by the majority of the population, while parading Marines in BD may be quite impressive)
- Support to planetary Armies, mostly as Cadres
See that about point 3 there may be some discussion, as while I:E depits all marine units as jump tropos, FFW doesn't explicity say so, so, folloint FFW RAW, no Marine unit (except 6/4518
th battlaion) is jump troop capable on it :CoW: (personally I think it should be considered an errata).
For the rest of tis roles and missions, Infantry is what you need, so my guess is that is an Infantry heavy forcé, with a small support branch and even smaller Cavalry and Artillery ones. Most support and artillery roles are taken up by the Navy, acting as its bases, supply and Medical branches and artillery (either with Ortillery and Fighters).
If we see what skills are in the Support MOS, we find that mostly they are in engineering (Cbt Eng) repair (mechanic and electronics), transport (vehicle), medics and Intellignece (computer) (CT:LBB4/MT, thpouhgh in the latter computer is replaced by the cascade skill Technical), IMHO reinforcing that they relly on Navy for support, most of this support being directly combat-related (except intelligence).
To be able to perform most those missions, they need to be equiped with the best equipment they can. So, I guess their standard TL is 15 (as all Marine units are in FFW, in I:E I guess they are still transiting form 14 to 15) and combat equipment is likely to be BD and lasers/gauss rifles/FG (I don't believe the FG to be the standard weapon, as it is too destructive if your intent is not a punitive raid, mostly in MgT wher there's radiation hazard, and lasers have the advantage of being Zero-G weapons).
I'd also expect all of them to be suberbly trained (and they are depicted as such in most texts), but (to my surprise), only a small part of them are considered "Elite" in both games.
While they seem in many aspects to be based on the USMC, I do not believe there's really an Earth equivalent, as there are too many different environs ans situations they cana find themselves on, far beyond any current tropos expecttives.