I managed to put Peragus back onto the Galaxy Map for my project, seeing as there are no more planets to fill that last slot I will leave it as it is for the moment.
It is the last working planet slot though, two of the 16 slots don't seem to actually function, that is currently Onderon, though I think it could be made to function it is locked out for the story and the Ebon Hawk which is currently used for Malachor V as the player doesnt need to exit to it.
Beyond that as far as I can tell it's impossible to add any more planets, unless replacing Peragus or Onderon.
Though there really is no need for so many planets, I just wanted to push the galaxy map scripts to their limits and see what could be done.
I think it is hardly noticeable that the galaxy map is not present during the tutorial, but let me know what you think if you are reading this extra long status update.