Missile troop order/protection questions

[Salamander8]

When you have a unit of missile troops, and you set them for say, Fire Fliers as an example, if your opponent does not possess said target, what do the missile units default too? I was thinking it would be: Fire Closest, but was not 100% sure.

On a related question. How do folks here generally defend the less armored, and normally rearward placed, missile types from fliers or fast ground forces that come after them? I was considering diverting some melee troops to the same rough area as the missile troops I want them to protect, but none of the orders seem like they would keep the melee troops covering the missile ones.