Anybody using Ground Combat/Units??

[Master Belisarius]

The Troops are only a minor feature in SE4.
Yea, you can design your troops (and it's cool), but you can not do so much...
In fact, the SE4 ground combat (like most of 4x games in the space), it's very simple: only one stack of invader Troops against the defender's stack.
In my opinion, I believe that MM did the right decision: the core of the game is the "Galactic Conquest" not the "Planetary Conquest".

About your questions:
1) The Troops, like the Satellites, Plataforms, Mines, Fighters, etc, are placed in the Planet "Cargo Area", after you build it.
You can load them into transports if you want, and "regroup" them... but the game always group your units, depending the name that you used to create them.
If in the early game, you build light Troops with Small Rockets, you should name them "Light Rockets 1", for example. But, if later, you have new tech to design better troops, you should create a new Troop design (with a different name), using the new tech.

2)I think that MM only was keeping the same idea than SE3...

3)I'm not sure, but think not. Anyway, I'm sure that the Troops are expendable resources: you can build a lot of them, very fast.