Would you read a book with this plot (assuming of course it was well written...)?

By the time you made it realistic enough to be believable (because you have to keep in mind how they would interact, political stuff, racial tensions, histories, etc.) I would imagine it would be very long. Keep in mind that there are hundreds, possibly thousands, of mythical creatures from all different cultures in the world, and I think it would be very difficult to include them all in a way that they were all important to the story and not just for the hell of it. But if you really can do it, more power to you. Assuming it was well written and had a captivating story, I might read it.