For things you describe at items such as swords, potions, helmets as well as NPCs I would not consider them as Stores I guess I would implement them as models. I, personally, use Stores as a way to collect some state outside of React's Component tree. I can have Store which has observable attributes which hold a list of Models, inventory for example and so on.
What about Combining stores it really depends on what you want to achieve. If you need to do something as a result of something has happened on one store, for example, a user has moved and Position store changed coordinates and you may have some other store which should react to this changes in some way and may use reactions for this case.
Another case might be that you just need to recompute something because some Store's value you depend on in some distant store has changed and you need to have different values now you might consider to use plain computed properties
If I didn't answer your question please post some real case scenario you want to implement and maybe I can help you with that.
Nick Luparev
front-end developer working with react/redux/elm