The pyreflies reacts on the persons thoughts. You don't have "personal pyreflues.
Or, as in Tidus' moms case, they willingly accept death and don't need the sending. When a person is sent, they cease to exist and only memories of the person remain. Pyreflies is not souls, it's more a lifeforce, a source of energy that keeps things alive. An energy that can be exploited as Shinra learns to do.It is a peaceful and calming land of the rested souls... If you think of a person there, and their pyreflies do not form, they have not been through a SENDING by a summoner, or they are not dead, either that or they are a fiend.