would you say that economic victory in a decent multiplayer game with 8 people is too easy? 

same goes for science and wonder victories.

is it possible to have a long mp game that lasts more than 200 turns if one or several players are skilled and pursuing one of the non military mentioned victories?

or constant bashing and raping of said player is needed in order to prevent him from doing it before 200 turn?

Just disable the economic victory type in the settings.