One of the best ground combat models which would work great for the SE series is the old Pacific War model by Gary Grisby. This classic was released in 1993 or so, and is now free for downloading at
www.matrixgames.com.
Translating the pacwar model to SEIV (or maybe SEV<g> ), here is how I would approach it.
The basic ground unit would be a division. Of course, this name could me modded to pretty much anything, such as legion, corp, herd, or whatever.
Empires would raise troops from population (exceptions could apply of course for special races), these would fill the division� ranks. Equipment for the division could and would me upgraded; equipment would include troops personal armor, small arms, artillery, tanks, and so forth. The equipment would be produced similar to the way troops are now produced, but once produced, be treated like stored minerals.
This pool of equipment is directly from Pacwar and its cousin, War in Russia (also available for free).
Divisions would remain and not be scrapped to upgrade, but when ready, there would be an equipment upgrade. Of course, the training level would drop as the troops would have to be retrained in the new equipment.
Supply should also be critical for troops, and I would increase its importance for both ships and troops; not for micromanagement purposes but rather to give greater depth and meaning to the space combat. After all, the SE series is really about ship to ship combat. But I digress.
Ground combat should be designed to Last more than one turn, the average combat should be designed to take approximately 10 turns, provided the ground troops can receive both replacements and supplies. That in turn, results in fighting for control of the space around a planet becomes critical, cutting off the supply source would lead to a huge advantage on the ground. Of course, maintaining a tight blockade should be much harder than simply winning combat around the planet, and in fact, could give rise to a whole slew of new combats; blockade running.
If only I knew how to program.