To be honest, since you have to complete all of the classes, it doesn't matter the order in which you complete them.
As for the fastest way to level, if you simply do the main story line and add side quests as needed, you should be able to level up fairly quickly and be ahead of the planet level. With the level sync, you will never get to the point where you lose xp so just keep playing on.
The main companion quests vary, but are all fairly similar in length. With the companion conversations being tied to planets now, not levels, it will still be a grind and take time to finish.