How to change the StreetPass message

The friend function is unrelated to the streetpass plaza. One person on my friends list has only appeared once in streetpass plaza - which was from when we set it up to begin with.

Miis can come to the plaza from playing multiplayer with them in certain games, like Mario Kart 7. Note that these can not be used for the streetpass games.

Other than that, you need to meet them "on the street", as far as I can tell.

There is no way to predict if they will scare your Street Pass Miis. Generally if the Mii is level 5 or higher they will not be as likely to be scared off by mummies (unless it is the Marauding Mummy, who tends to scare all Miis).

The best way to get past them is to make sure you have more than one person in your team - they will only scare the first person they encounter (one per mummy), so then you will have people to attack them still.