Q: Can only scouts scout?
A: No. Any unit can scout
But scouts are the most effective, since scouting is a function of morale + speed. And scouts have good morale and superb speed (they can even count as mounted and not mounted, whatever favors their movement the most). AND they have an extra scouting bonus on top of that.
The lighter types of cavalry are also good for scouting, outriders in particular, but dragoons and lancers also work. In very rugged terrain nothing beats skirmishers for scouting (since cavalry is so hampered).
Another good thin about scouts is that they can go into a province, scout, and not even be detected. And even if they are detected they have a good change of disengaging and fleeing.
Cavalry is less able to stay hidden, but good at withdrawing.
Skirmishers are OK at staying hidden, but not so good at withdrawing.
Scouting is also about numbers - the more units you send in, the bigger the chance you'll learn something. Here a scout unit can truly shine, since it is the most economic way to scout - one scout unit can provide as good intelligence as several outriders for example.
The only real drawback with scouts is that they have limited combat potential, and they they are easy to destroy. If they are detected and don't manage to flee, then they are dead. A bigger cavalry/skirmisher force has a much better chance of surviving an engagement with the enemy, perhaps even doing some damage in the process.
When employing scouts there are two paths to go down; single elite+ scout units or multiple green ones.
