How did the ancient Persian Empire connect so many different peoples?

Imagine you are an emperor, and overnight your kingdom becomes the biggest the world has ever seen. From Egypt to India, hundreds of peoples, dozens of languages, gods, foods, and customs — all suddenly yours. The Persians faced exactly this around 2,500 years ago. So how do you hold a thing that enormous together without it falling apart?
Their first clever trick was to NOT make everyone the same. Many conquerors smashed local customs flat. The Persians did the opposite. Worship your own gods, speak your own tongue, keep your own laws. As long as you paid your taxes and stayed peaceful, life looked much like before.
