Total number of Sith Lords? Hm... it changes over the course of the game, what there is at the start isn't how many there will be in total by the end. I'll leave it at that for now.
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The fighting stances? Some are focused around tuning your lightsaber use while others are better for those that rely heavily on the force powers instead of lightsaber combat. So different classes will benefit from different stances.
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I talked to the designer in charge of making decisions like that. He said that if NPCs could be used to pass every skill check, then there would be no point in the player character leveling up their skills themselves. Whether or not one agrees with that point of view, that's how it is currently. There's some instances where only the PC can make the skill check.
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You won't be able to select tattoo's, but that doesn't mean some characters might not end up with them, depending on how things go.​