As for strict experience point levels you would have two levels of rogue and four levels of monstrous humanoid. Whenever you have racial hit die most of the abilities of the monster are tied to them. So, you could technically fore go the racial hit die but you would have a useless doppelganger and lagging rogue. As for balance you could afford him to have a couple more levels of rogue. I was never enthralled with the doppelganger when you could easily have the polymorph spell or be tearing someone's face off with a druid or werebear while he is um.... pretending to be a low level race.