The only things needed to know the rules of a particular card are just a rulebook and the card you are talking about.

Regarding _EX_ cards, there's nothing written on them about limiting them to one so the rule of _"max 4 pieces of cards with the same name In deck are allowed (except Basic Energy)"_ applies.

Also the text _"Evolves from X"_ on Evolution cards (= not Basic) is important because it's the name of the card you can only play the Evolution card onto. In your case, Lucario-_EX_ is Basic, _M_ Lucario-_EX_ is Mega evolution that "Evolves from Lucario-_EX_" so you only need Lucario-_EX_ in play to be able to evolve to _M_ Lucario-_EX_, any other combination won't work.

In tournament, as long as you have at least one Basic Pokémon card in your deck, you can't be punished for playing an impossible evolution cards, you would just have cards in your deck that you are not able to play under any circumstances. On the other hand, don't forget to check the actual format of the tournament, all premier events nowadays use either Standard format (Sun & Moon and newer) or Expanded (Black & White and newer) and your _M_ Lucario-_EX_ is legal only in the latter one. If you showed with _M_ Lucario-_EX_ on a Standard event, than you would really have an illegal deck.

As you pointed out, the difference between Pokémon _EX_ and Pokémon _GX_ is that the former ones are always Basic whereas the latter ones are always the stage they are supposed to be. However this difference is conceptual and has no impact on the game itself. You just read the cards and you are fine.