[:)] How is the grond action, is their individual combat units per planet and are there any contoll over the combat action. Can each planet have diferent ground troops and air defenses?
How do you take an enemy planet, with ground troops along with air support and planet bombardment ?
As a rule, yes.
Early on, taking colonies often only requires destroying their defenses, as they'll rarely have much in the way of ground troops (aside from perhaps a few Militia units). Later in the game, however, a combined-arms approach like what you describe is generally more the rule.