If you can't get a group together to finish a Heroic or Flashpoint, wait till you reach the level cap for that planet and go back and complete them while solo. For example Korriban is levels 1-10, wait till you reach 10 and go back to complete, but if you complete it while your above the cap say 11, you will miss out on the experience and loot. You can even try it at level 9 just to be more safe. For the flashpoints I would just go with the level cap from the next plant in the main quest