"and known among your tribes": They should be familiar to you. If someone comes cloaked in his talith and sits before me, I know nothing about him. But you know about him, for you grew up with him. Thus, "known to your tribes." R. Shimon b. Gamliel says: No (judicial) session begins until people bring it into question: Why did this man consent to sit (as a judge)? And why did this man not consent to sit?
"And I shall set them at your heads": For you might think that if you appoint them, they are appointed, and if you do not appoint them, they are not appointed; it is, therefore, written "And I shall set them." If I appoint them, they are appointed; if not, they are not appointed. I might think that if you have accorded them greatness they are great, and if not, they are not great; it is, therefore, written "at your heads."