The Japanese Special Naval Landing Forces were closer to a U.S. Marine Defense Battalion than a standard Marine Battalion Landing Team. They did have a couple of rifle companies, but they also had a fairly large heavy weapons unit as well, along with support units. The unit that was to land at Midway was actually an army regiment trained in amphibious warfare. That was the force that the Marines pretty well wiped out at the Battle of the Tenaru River in August of 1942.
For those interested, you can find the Handbook on Japanese Military Forces at archive.org.
The SNLF at the beginning of the Pacific war were simply naval infantry battalions composed of sailors seconded to be infantry. As WW 2 progressed, they gained more heavy weapons and such becoming base / island defense forces of roughly battalion size.
They were in no way elite or specially trained for their mission.
The Ichiki detachment you mention were simply IJA infantry that were designated for landing at Midway and when that operation failed, they were sent to Guadalcanal and wiped out. Their only "elitness" was the unit had previous combat experience in China and was supposedly extremely good in combat by IJA estimates.
As for Imperial Marines in
Traveller, I see them as pre- WW 2 US Marines or closer to Royal Marines. That is, they're true naval infantry. They're not used in huge numbers normally. Their purpose is to defend the ship if boarded, and to stomp on the natives, so-to-speak, if they get uppity in some system. If they need heavy weapons, some of the ship's crew is assigned to man those to support them.
Equipment-wise, they would have power armor and weapons suitable mostly for a fight in space and ZG. If they have to land, they use the same weapons, or are issued ones suitable for the world they'll operate on.
I don't see them operating in divisions and corps. That's what the Imperial Army is for. The Imperial Marines aren't the cannon fodder the Army is. They are better trained and equipped with expensive weapons and personal armor systems.