What we are meant to do is remove the stock IP world and replace it with EllaCharmed's. We can't have both of them in play at the same time, they would conflict with each other. If you did not remove the stock world first because you did not see it, then you were in the wrong place within the Program Files. Unfortunately I doubt that her fixed world will have any impact on mermaids, though.
And I would strongly advise adding in MC Cheats to fix the mermaid issue. The add-on doesn't really add much in the way of measurable overhead to the game, in fact none of MC does. It's true that it's a very complex mod, but once the game loads up it doesn't really have anything to do until the player calls upon one of its menu commands unlike, say, StoryProgression that is running and using up game resources all of the time.