Typically, you have to encounter everything that you roll, regardless of its relevant to your current plot. This means that every pokemon you roll, you MUST encounter in some way or another; but they don't necessarily HAVE to be enemy pokemon, though they typically are regarded as such. As for items, it's easy enough to just roleplay that your character found an item lying around in some dungeon or another.
As for whether you can encounter another person's roll; it's generally etiquette to at least talk to the other rper about whether they want you to encounter it or not.