NOTE: the quotes are from another thread, but I guess we'd better keep the discusión on those ships here.
I'm thinking your transport would hold closer to one of today's Battalion Landing Teams. The Marines aren't going to land just a rifle battalion. They'd have an attached artillery battery, tank, LAV, AAV, and engineer platoons, air defense and electronic warfare sections, and an MP squad. Plus whatever logistics units are attached.
If it's just a troop transport for follow-on troops, they could fit two TL 15 battalions with room to spare. But, if it's an actual assault ship, the battalions would be task-organized.
I don't believe they are the equivalent to those Battalion Landing Teams, as, as Timerover rightly points, the transports are not able to land the troops by thmselves, but they need the Command and Support ship.
About not landing just infantry, I'm not really sure about the Marine battalion TOE in the Imperium. As said before in this same thread, I just took the numbers given in CT:LBB4 for battalion and reinforced battalion (452 and 635 men, respectively), and assumed reinforced ones, and, again as said earlier, I guess most this reinforcment will be in engineering and infantry troops, but probably, as you point, some MP, EW and other specialized toops will be among them.
As for artillery, the fighters in the Command and Support ship and ortillery from supporting ships are assumed to take most of this role (remember, I always assumed orbital supremacy to begin the invasion, and that probably means intelligence and electronic support from supporting fleet too, and probably the reduction or neutralization of defending air assets by it too).
See that the fighters are likely to be quite good too against enemy armor (that uses personal scale weaponry and armor, against the ship scale from the fighters) too...
Your transport needs to have the landers carried by it, not another ship. The US tried that in the Torch invasion, admittedly in World War 2, and discovered that getting landing craft to the right ship was a bit of a nightmare. How many men can the landers carry? Multiple trips with the transport in orbit are going to be a problem, or is the transport essentially hovering in one spot with contragravity lift?
This may be a valid point, but I guess this will be more true for ships designed to act more alone. In this case, those are highly specialized ships never assumed to work alone, but in task forces and as part of an invasion fleet.
As for the problems you talk about in operation Torch, I assume that maneuvering (and docking) in space is easier than in sea, as you don't have to care about waves, tides and so on, and so that docking the landers to the transport ships for troops transfers will be easier than for the landing ships the wet navy may use.
As said in the post presenting the Lander (#4) they can carry up to over a full such battalions (670 men, at 3 men/ton) in a cramped way with no accomodations. As I said, they should be seen a the landing crafts seen in the first scenes of
Saving Private Ryan film, with the troops cramped on them and ready to land as soon as they hit the ground (or even earlier, if they are grav equiped).
And yes, the trasnports are expected to be in orbit (and far one, to avoid fire from the planetary defenses), and the landers are expected to perform several round trips to land more troops, and even to also be used for supplies from other ships, as the LTVs did in Tarawa. They relly in their high speed and heavy armor to do that, though probably there will be losses and their numbers will be reduced along the trips.
Your cargo space is borderline for that number of troops. A commander is going to want as many supplies as possible landed with him, as additional supplies are going to have to come from another star system, with no guarantee of arrival. I would agree with LiNeNoiSe that you are going to have one battalion and supporting troops carried, and not two battalions.
Yes, the space is quite at premium, and troops will travel cramped on the ships, but I believe the 2 battalions/ship are posible in such barracks. They are not expected to stay in the ship for long time, just for the transport to target system (be it the one to invade, to garrisson, etc), not to stay there for long time as ship's troops.
About supplies, again remember those ships are not expected to act alone, but to be with a larger fleet where supply ships (and probably second wave troops, incluiding most non infantry ones) are. supplies and more troops can be carried by any cargo ship, where the landers may also dock to take them to the "skybridge" if landing them by other means is not safe enough.