Remember, each SR "Unit" contains multiple vehicles - an infantry unit usually contains a battalion of 54 APCs, Engineers, whatever. A tank unit is actually a tank battalion of 44 tanks. It is easy to exceed the bounds of reality in the game - it you have over 204 M1 Abrams tank units then you own more M1 tanks than have actually ever been built in the real world.
Not sure about the role of recon/commando units in the game (need to revisit that part of the manual). I understand from the manual that they can "go stealthy" if they are entrenched, so I would think they are good for setting ambushes?
On the Unit Command panel, first select a unit and then go to the Individual Rules of Engagement sub-panel. There you can toggle the button that says "Unit set for Stealth Approach - No Land Captured". This is useful for having engineers use Demolition against enemy facilities or when parachuting commandos into enemy capitals. If the commandos are set for stealth then the resident enemy garrisons will not "pop out" when your commandos land.
btw: Recon units are not commandos - they are long range recon units that are not as good fighters as Marines, elite Airborne or Engineers. They can spot and travel farther than their infantry counterparts.