i kinda had the same problem, I went to Nar Shaddaa before I went to Taris and ended up being several levels about where I should be for Taris, but it all worked out because I got no xp for the mobs and quests were only giving 5xp so by the time I hit the end of the Taris storyline I was hitting quests and enemies that where giving me trouble. So basically being overleveled may hurt xp in the beginning but you and it do meet up in the middle along the line