Being as you the tank commander are defining the mission - I don't see why you shouldn't be able to select your own mix of rounds from the "ammo depot" up to the maximum number carried by the tank. Start out with a "default" loadout and switch out rounds as needed.
While I don't know specifics, I can vouch for the tank commander NOT being the link in the chain the decides how much of what kind of ammo is taken per mission requirement, that is decided higher up yet. Other than Pieper and the Battle of the Bulge, I'd like to think SOP was followed in most situations. There are all kinds of mission parameters that come in to play than just what the defining mission is. For instance, I can vouch that if there is a company of tanks that is going to be moving 60 miles in a "sprint" to help defend an area from an impending attack and it will be three days before resupply is available, you can count on two things: more than the standard ammo load will be carried (external), and the mix of ammo will be defined by probable enemy forces. There are reports of US tanks in Korea coming to the front to fend off the Chinese horde with nothing but a few HE rounds and thousands and thousands extra ammo for the MG's. Evidently the MG's were found to be more effective fending off the hard charging Chinese than HE rounds fired once every 3-4 seconds. But, in terms of WWII if you're in a Panzer IV H and you on the outskirts of Berlin and you're sent to hold back the column of T34's racing through a valley, odds are you're going to take as many AP rounds as you can with a small load of HE for any enemy infantry that may show up.
I for one think that tanks should not be able to take 100% of anything. I think there should be 4 options of various round counts, similar to what HTC did with the jagd panzers, but allow for more versatility. Only the designated TD's should be able to carry 100% AP rounds, and the LVT4 and M3/75 be able to take 100% HE.
