Anyone can take them on because it says in the simulation that you can use a character to their fullest potential with any custom jutsu you have created for the character. It's called a simulation so you are allowed to pull out all the stops.
I plan on having Mio, my Genin, face one of the Kages at her fullest with having utilized every single one of her bloodline jutsu that I have created for the Shoubu Clan. It's just a fun way to get everyone to push themselves, in game, to get their characters ranked up and see just how strong their characters can become. I also am going to have Ahira fighting as well.
Again, this is just for fun. You can even have one of your characters die. It's a simulation, nothing in character. Think of it as filler, if you will and a way for us to simply just fuck around and have some fun.
P.S. Besides, you GET EXP FOR CRYING OUT LOUD!