Kind of a double standard to only apply type advantages that benefit ash doesn't it?
You don't know that hawlucha will be used a lot or that charizard will only be used at the league. Mega evolutions play too huge a role for it to end up like that.
There are some electric pokemon in the gym, doesn't mean clemont will be using them or even own them.
Magneton and Heliolisk are almost guarantees to appear going by previous Gyms. Also, none of Ash's Pokemon do have type disadvantage for the next Gym.