In the online league the leaders were separated by 'ranks' that contained (I think) three leaders per rank. The challengers could fight any of the leaders in their current rank and could only fight leaders from higher ranks when they had defeated all leaders in their current rank. So I guess in-game the ranks would be something like this:
Novice: Julia, Florinia, Shelly
Intermediate: Shade, Aya, Serra
Advanced: Noel, Radomus, Luna
Expert: Samson, Charlotte, Terra
Elite: Ciel, Adrienn, Titania
Champion: Amaria, Hardy, Saphira
This is soft confirmed by the fact that only Leaders ranked elite or upwards are allowed to use Mega Evolution and the first leader to use a Mega was Ciel.