Mixed units actually exist in Clash. A unit is made of elements, and you can have various elements in a unit. For instance, a tercio unit could be made of 500 pikeman-element and 500 musketmen-element.
The unit is depicted as a single image in order to fit on the screen, but a detailed battle view could of course show the elements, so each element would need its own sprite.
In terms of tactis, I'd rather have orders given prior to the battle (a la Dominions if you know that) and a battle view would only be a way to see what actually happened.