- You can write about already established characters if they were not regulars in the incarnation you have chosen. This is straight-forward for something like TNG. You can write about Sito Jaxa or Nurse Ogawa. Rule of thumb: If their name appears in the opening credits, they cannot be the protagonist.
In DS9 terms I guess this gets murkier, it means you can use Garak but not Jake...well that seems fair enough![]()
DS9 is a bit trickier. But I would say the regular guest characters are also out. After all this is about the folks who don't get to be in the limelight. Use a common sense approach, somebody like Garak who ends up helping to liberate his home world is not a small fry.