1) No. There was no sexual reproduction. Rahi did not have "young."
2) Yes. Just as Matoran were capable of drawing energy from fish, etc., Rahi could get it from other Rahi. And since they didn't know they could do it just through touch, sometimes they ate them. If, for example, a certain breed of Rahi was needed in large numbers at times for a particular reason, having a predator for that species would help keep the numbers down when they didn't need to be in such abundance.
3) He did, but I don't have any list of which Makuta created which Rahi.