[@o3o]Mega-evolutions are allowed, but keep in mind that during the League in Evoru the Pokémon must be fire, water, flying or ground. There is an instant disqualification if that is no longer the case. In gym battles this restriction doesn't exist, there you can use any type you want.
 
And Pokémon don't forget moves, you may use any move that is reasonable for the level they are. I don't ask people to count levels, but a newly caught Pidgey doesn't know Wing Attack. Try to keep it realistic, based on the Pokémon Universe established in the anime. 
The RP aims to emulate the anime more than the games, so battles are also done more anime-style. Feel free to use the surrounding to your advantage and use attacks as a counter attack. 

I should add this to the first post, but unless a Pokémon can hold on to the item, or carries on them as an accessory or a piece of clothing, items during battles are not allowed. You can't run up to a Pokémon mid-battle to apply a potion, but if the Pokémon wears a bracelet or a scarf with a megastone or another item that has to be held, that is allowed.