Perhaps the first pokemon you send out is always the one who is at the front of your roster in the pokemon line-up in your post before the battle starts?

The same way it works in the games, sorta? I wouldn't mind it, and it realistically could reasonably be the first pokemon on your belt/wherever you keep your pokemon, so in a sudden battle situation it's likely to assume you'd grab them first.

Just my thoughts, though.

But yeah, only being able to switch if it's a relatively safe environment (not being attacked at the moment and stuff like that) Makes the most sense. It'd be sort of hard to recall and send out pokemon like that.