The order "Espionage" for an army is designed to spy on enemy colonies and to being able to estimate how strong a resistance is to be expected.
For this mission, only spies can be used. No other units qualify for it.
Spies have to fight against the counter-espionage spies of the target colony, and will only be able to produce an usable intelligence report if they are superior to the spies in the target colony.
The loot potential always refers to the stores of the target colony.