Nope. Standard battle procedure. No reason to limit levels. Remember, the gym leader/elite 4 may end up constantly cycling around due to rank challenges, and whatnot. If we started switching gears on peoples' levels for the sake of authenticity, that might end up fucking with several of the leaders' heads, and making the league a giant clusterfuck of confusion.
As for the mega evos, we /could/ do that. The problem is the single-type clause would severely hinder the ability for multiple choices of some of the members if we do it like that. In the example you gave, this would prevent the Dragon user from using: M. Ampharos, M. Sceptile, M. Charizard X, plus anymore that turn into the dragon-type in OR/AS, which if there are no more Dragon M. Evos (Unlikely, but let's roll with it for now), leaves 3, and only one gets a type change. (Altaria) Granted, in theory, this can still be worked with, but it does really limit the choices the members have in an area where they already are pretty damn limited.