unless I'm vary much mistaken, only about 40% of a large military unit is comprised of combat troops, the rest is basically non-combat support, right (or do I have it backwards)? so I can see why a 30% loss would cause combat ineffectiveness.
Depends on the unit. Some formations are comprised of one hundred percent combat arms personnel. They're typically divided at the regimental level or lower. They're a composite group at a brigade or higher. Of course, this isn't always the case.
Without going nuts on details or specifics for each branch, you have your breakdown of the basic chain of command:
Individual Soldier
Fire & Maneuvers Team (2 Soldiers)
Fireteam (3-5 Soldiers, typically 4)
Squad (7-12 Soldiers, typically 9 in the Army)
Platoon (26-64 Soldiers, typically 45)
Company (2-5 Platoons)
Battalion (2-5 Companies)
Regiment (2 or more Battalions)
Brigade (2 or more Regiments)
Division (2 or more Brigades)
Corps (2 or more Divisions)
Field Army (2 or more Corps)
Army Group (2 or more Field Armies)
Army Region (2 or more Army Groups)
Theater (Region of special interest)