Calling Ammon “the chief” among them provides a further suggestion that he was the eldest, corroborated by his being listed first when the sons are named (Mosiah 27:34). As noted in the commentary accompanying that verse, the only counter indication against his being the eldest is people’s initial proposal that Aaron be the king.
Mormon provides no description of Ammon’s administration, but it obviously involved a spiritual dimension of blessing them, not merely making decisions and giving instructions. Ammon must have understood Christ’s style of leadership: “But he that is greatest among you shall be your servant” (Matt. 23:11)—and in fact, his mission among the Lamanites corroborates this approach to leadership.