For purposes of education, I recommend a wild course of reading primary and secondary sources from all over the world.
Nevertheless, for purposes of writing, I recommend if a writer is looking to
steal derive his creatures, magic, and gods from folklore, he should settle down, pick one culture (per story, not per lifetime), and settle to work with it. Like J.R.R. Tolkien's Northern European derived mythos.
(That is, after the writer's decided that the question this elf necessary? has an answer of "Yes")
There are any number of fantasies with a mad jumble of creatures: banshees and sphinxes and fox spirits. But such fantasies often lack unity. Possibly the knowledge that they come from different cultures doesn't help, but discordant monsters often seem to come from different universes, even in the same work. The philosophy from their native cultures have colored the tales of creatures, and so their character. The code of honor of a fox spirit and of a fairy is not the same.
It's not so much a realistic jumble of random elements that lends depth to the world, because they don't cohere. In some extreme cases, it rings of random-generated dungeon crawls from D&D.
To be sure, if the story aims for nothing more than a high-spirited piece of fun, it can work.
And sometimes, such a jumble can work as a serious piece of fiction. However, for this to really work, the writer has to structure his world to take them all in. To heat them all to the melting point so they do not remain a jumble but fuse together into a new structure.