First off, I'm talking about a true, professionally trained army. Humans also learned, very early on, that they are not alone and the universe is very Darwinian. Also, human nature doesn't change, even after going into space. We are still an argumentative bunch that doesn't really turn the other cheek. EDIT: There are multiple threats; other humans (piracy, etc.) and unfriendly aliens on the fringes. The human threat would normally be handled by the marines, but responding to attacks from the aliens who may want a planet back, would be the army's responsibility.
In my universe, the marines are meant as shock troops that ship out with the naval units. In case of need, a fleet would ship out with a transport with the heavy equipment needed for an assault and the ship board marines would be combined into a fighting force. Much like the US Marine Corps, these troops are meant to take a beachhead and allow the army troops to come in and continue the fight. Travel in the universe between stars and in Einsteinian space still takes time, so deploying from a single planet could take months or even years.
How then should the army be structured to maximize training efficiency and minimize deployment times in case of trouble?
I'm looking at the US Army as model for my army. There will be troops trained in combined arms tactics and in specialized fighting techniques. By that I mean that a typical regular division would be mechanized infantry with armor mixed in at the regiment level and the requisite support units to allow each to deploy on its own. It doesn't make sense to me to keep enough of the heavy lift ships to deploy all mech. infantry divisions at once. I would only want to deploy 25% of the divisions at once.
There would also be specialized divisions such as full armor, cavalry scouts or light infantry as well. These would be scattered throughout the occupied space on a regiment level. The regiments would also have the specialized lift ships on hand to deploy.
Since the typical division is mech. infantry, deploying one would require sufficient heavy lift capacity. Should I have pre-deployed equipment that the trained soldiers could fall in on much like the POMCUS sites planned for use in the REFORGER operation. This equipment would be on heavy lift ships in orbit near likely trouble areas and maintained by civilians. This would minimize the need for heavy lift capacity but the response time would not be quick. However, these troops would be highly trained and motivated.
The other option I'm thinking of would be like the National Guard model. A division would be on each planets in the federation (or whatever) and the units would be a mix of professional cadre and reservists. Many of both would be recently discharged marines and soldiers. This would maximize the response time, but the heavy lift capacity needed would be extreme. Additionally, like the National Guard, training to standards would be an issues as well.
Post Answer Edit: I read abot 50 pages of Starship Toppers and Rolling Hot by David Drake last night. I think I'm going to go with a hybrid strategy. I'm going to have several main bases scattered through human space. Additionally, I'll have several forward deployed regiments near the frontiers. For deployment, I'll have a fleet of heavy lift ships that can ground and carry an armor company each. During "peace time" these ships will be psuedo-civilian conducting trade between the bases and the frontiers.