^That really is a good point about Lamias master.
Never thought about that one myself.
Since you said, its not a definite rule, there's really no need to post more.
But I just want to post 2 another examples of not needing to be the strongest wizard. That I can think of.
When Makarov wanted to appoint a new Guild Master for Fairy Tail. He considered Laxus, Erza and Mystogun.
Laxus is way stronger than them, yet, he still had others in mind.
During the 7 blank years.
Makao was the leader, but there were still (presumably) wizards above him.
Wakabe beat him in the Fatasia-arc
Max was on holding his own against Pre-Second Origin Natsu.