FFX has a very linear plot, it would be easier to script. I can't imagine how much you'd have to cut out and add to others to make the story appear fluent in a film. FF8 wouldn't work, too much of the storyline is moulded around giving you another boss to fight or another mini-quest, with non of the mini quests or monsters making too much of an impact to the rest of the game (which is important in a film), until you discover the big plot twists, the biggest of which has been used so many times in films it would make the whole thing look cheap.

FF8 would work better as a TV series, each episode with a SeeD mission and a monster which doesn't need to be remembered in the next episode and a long term plot building up the same twists in the game.