Were concentration camp Kapos sometimes Jewish prisoners, or were they always non-Jewish convicts and political prisoners?
Some of the concentration camp Kapos were indeed Jewish inmates; others were not. Regardless of their ethnicity, all of the Kapos were under pressure to perform whatever function was assigned to them due to the numerous possible punishments--one of which included simply being nailed to a box and left for dead. This, however, did not stop the prosecution of Kapos as war criminals following the fall of the Third Reich. During the trials, the Kapos were treated as if they were complicit in the actions they had carried out in the camps--a dilemma that still haunts Holocaust studies.
The point of the Kapos was, of course, to instill constant fear in the inmates, even of each other. This also took place outside the camps with the use of Block Wardens, those who were lowest on the Nazi hierarchy and responsible for reporting any "suspicious" activity. But anyone could have been an informant. In fact, people sometimes reported others simply to be rid of annoying neighbors or competing small business owners.
Both. Kapos were the head of the blocks. Alphonse is the head of Elie's block at one point in the story and as stated in my book (HRW) on page 59:
We has left the tents for the musicians' block. We were entitled to a blanket, a wash bowl, and a bar of soap. The head of the block was a German Jew. It was good to be under a Jew. He was called Alphonse. A young man with an extraordinarily aged face, he was entirely devoted to the cause of "his" block. Whenever he could, he would organize a cauldron of soup for the young ones, the weak, all those who were dreaming more about an extra plate than of liberty.
Obviously the Kapos who were not Jews were considerably less compassionate.