Personally I don't see the TC marines having heavy equipment. Let me explain my view a bit. The TDF (particularly before the RW) has a lot of similiarities to the pre-WW2 US military (especially the pre-WWI military).
The US military at that time was composed of a large navy and a small army. Having few neighbors (only Mexico & Canada) and few colonies/territories (Philipines, Hawaii, Guam, Wake Island, Midway, American Samoa) the US had little need for a large standing army, but it did need a large navy to protect it's merchant traffic from pirates and to maintain Sea Lines of Communication (SLOC). As a result the navy was large with modern Battleships, Aircraft carriers, etc, while the army was a small organization that during times of war would serve as the officer and NCO core of a larger force composed of National Guard units, volunteers and draftees.
Periphery 1e (p.28) specifically cites this as the Taurian model - the reason for Case Amber was to have the navy buy time for the TDF to integrate reinforcements training in the Hyades Cluster.
Using the US military pre-WW1 model for the TDF Marines are mainly guards for the navy- they are assigned to defend naval installations and ships from attacks, lead boarding or shore parties and fill secondary functions on vessels (including damage control and AA support). The Marine amphibious capability (which is what it seems you are going for) did not begin to really materialize until after WW1 and came into full blossom during WW2.
So I see the TC marines serving mainly on Taurian vessels and facilities as guards and conducting boarding operations against enemy ships.
The other reason I don't see the TC Marines filling the assault role is because the TDF already fills that role. The TDF has 3 levels- the Noble Family regiments (aka national guard/reserves), Regular Army and the Battlemech Corps. This is hinted at in the source books but since only battlemech forces get fully detailed it is somewhat overlooked.
The Noble Family regiments are the reserve of the TDF- they are only called up during extreme emergencies and serve to defend their home world. The regular army is composed of all of the Armor and Infantry regiments not assigned to support battlemech units- Remember every TC has at least one Armor battalion, with important worlds have multiple regiments. These form the bulk of the TDF and plain the major role in defending the Concordat (remember prior to Shraplen coming to power the TDF was a DEFENSE force not an offensive one). In a defensive role tanks and infantry are very effective - the battlemech's major advantage is it's mobility both tactical on the battlefield (being able to move through almost any terrain) and strategic (easier to transport over interstellar distance and the advantages of being able to conduct orbital drops).
The TDF's battlemech Corps are a cross between the modern Marines and a mobile defensive reserve force. If you need to kick in a door- you go to a TDF mech regiment. Need to rapidly reinforce a planet against an enemy invasion- send in a battlemech regiment. Also remember the TDF favored forming Special Task Groups- any battlemech units not actively involved in defending against a planetary invasion would be deployed in counterattack roles or as deep raiders.
Much like the US Marine Corps between WW1 & WW2 they need a distinct mission from the army-for the USMC that mission it was the Amphibious assault role- in the Concordat I think it is the space combat mission. So in my view- the TC's marines are light infantry, highly trained in zero-g combat. They are probably organized in battalions and even regiments but usually deployed in smaller detachments on board Concordat ships. The probably have aerospace assets (fighters & boarding shuttles) but I think mechs and tanks would be rare- maybe just a few battalions for use in assaults against targets located in the vacuum of space (think a facility located on the moon or some such).
That said I like your organization- I like mixing tanks and mechs at a low level and I love the idea of mechs organized into lances of 6 units and companies of 18. I'm just not sure I see this a Taurian Marine unit.
