As far as I know they are instant and triggered at the same , when the level is completely loaded.
I would like a confirm from a more experienced person too. But so far I got many BeingPlay events that are connected to each other on my own game and they are not having any problems. For example my Gamemode does something to my player right away, while my player is giving info back to my gamemode at the same with both beginplay events. I would have an error if there was an order.
One thing I am not sure is that, if there is an order and it is not obvious when the game is small. For now my game launches in 1 second so maybe I am wrong.
Best way you can check I think is, make Being Play event on all those classes, and connect Print Strings upon them with -different- colors. If the color is randomised everything you launch the game, there is no order. If they are always the same pattern, there is. ^^