Yeah, Mordecai suggests to Elincia that she just kill her enemies before they kill her, because in Gallia, you just have to be stronger than the king to become the new king. So it would seem that anyone stronger than the king can become the new king. And no underling would ever challenge the king, because they are necessarily weaker than the person in charge of them. As in, Kyza wouldn't be an underling to Ranulf if Kyza was stronger.