If you add the mod later on, all existing players and their buildings will belong to the "player" force, and not to new planetary forces introduced by this mod.
If someone joins or dies and respawns, they can select on which planet they want to start.
Selecting a planet you have not been on should work perfectly fine. You might consider restricting the selectable planets in the map settings to those planets.
But if they select Nauvis or Fulgora they belong to the corresponding "nauvis" or "fulgora" forces, not to the "player" force to which the factory belongs. That means, they can not use space platforms, rocket silos, radars, logistic networks and labs that belong to the "player" force. Their technology tree will start with nothing researched.
If some buildings don't behave as expected (like the ones mentioned above), you can mine them and place them anew, then the buildings will belong to the force of the player who did it.
(Note that this mod is considered to be a multiplayer mod and does not make too much sence playing in single-player mode.)
