So, basically, you know how the grenade quick-slot works? You right click it, and you can select which grenade to use.
Well, how about we extend that to ammo types as well?
Obviously, we can do that with the quick reload slot (for rockets, at least, right?) (If we can't, we should be able to. I simply can't remember at the moment. )
So, we have (or can have) the grenade quick-slot functionality in the ground combat GUI, simple right click popup box.
But then, what do we do for the main issue, equipping our troops with them in the soldier load out screen?
Simple, we do the same thing! Simply right click on the ammo in the equip screen, and you can select a new one that will then take the old one's place, until you right click it again and swap it for yet another one.
One idea, we should have the weapons in the load out screens be equipped empty as default (with a big red EMPTY sign over the gun icon so we don't forget to load it). This would allow us to load rocket launchers with whatever rockets we want initially, and if we have multiple ammo types we can select whatever ammo type we want to be loaded into the gun. Plus, this makes sense from an in-universe point of view, doesn't it? I mean, we don't store weapons in an armory with the magazines loaded in them.