How much space does ammo take up? 12 SCU doesn't seem like much for rearming.

That depends on what ammo you are bringing! The current volume of the cargo grid is 2x2x3 SCU which is a volume of over 20 cubic meters of space to fill with ammo, ship items or cargo. The cargo room on the Vulcan is also quite spacious and if during development we feel the need for greater capacity then there is room to increase this.