Partners are rather interesting in EDH, since they give you incredible flexibility in what you deck can do and which colors it can have. Companions are almost like a second commander, but have extremey rigid deckbuilding restrictions.
The challenge? Build a commander deck with partners as commanders and use a companion.
Lutri and Yorion don't work, and there is no Gruul companion, since Jegantha, the Sellspring is 5c. Good luck!