Right or wrong, was this way since RoP 1.0.
It is all about seniority, a lesser number means Prince Wilhelm is above Markgraf Karl in the ranking, and the game puts him in command.
Just checked, and historically Markgraf Karl Korps was commanded by himself, and the little brother of the King was not there.http://www.kronoskaf.com/syw/index.php? ... _August_26
He will only appear after the defeat in the battle of Kolin.http://www.kronoskaf.com/syw/index.php? ... st_Wilhelm
As you may see, there is a wealth of information in this site dedicated to the 7YW!
You may send him with his brother the king, like Narwhal did in his AAR, that is highly recommended if you are new to the game.https://forum.paradoxplaza.com/forum/th ... rs.513080/
Anyway you may rename any stack with Alt-Click.
Note the plural of corps (army) is also corps