Correct me if i'm wrong but once a Trill is joined to a host dont they become reliant upon the host and tend to die once unjoined which is why they have to go on being joined to new hosts once their current host expires?
At any rate i believe that symbionts swim about and reproduce and stuff in their underground pools and only join with a Trill once they have become bored of their existence within the pools since it seems like a one way deal. I have an idea of a symbionts first joining involving an unjoined Trill initiate walking naked into one of the symbiont pools and waiting patiently for a symbiont to come and make the life changing decision to join with them by swimming into their pouch. (The guardians who tend the symbionts and their underground pools would have informed the symbiosis commision that one was ready to choose a host).
Symbionts definitely communicate with one another through the electrical charges carried by the underground pools so they might know how they relate to one another but i like Timo's idea that they spawn like fish.