I just tried to unlock the Tython codex using my smuggler char and was only able to get the datacron, map location, and some of the bestiary codex entries. I don't think codex entries for starting planets are completely unlock-able unless your char starts on that planet. For example, I have done a Jedi and was able to unlock the entries for Mangka Cats and Flesh Raiders, but was unable to get them with my smuggler - I think those entries unlock via quests only. Also, my smuggler was unable to take on any quests on Tython, not even the daily Chamber of Speech.