Limiting the bonus feat selection to every other selection is probably a good one, though one option is to set a level where it starts, making it, say, only available after 10th level, or perhaps even as low as 6th level, will prevent people who aren't taking at least 50% of their levels in Noble from abusing it.
3 options.
#1) A bonus feat can only be selected if one was not selected for the previous special Ability Selection
2) A Bonus Feat can only be selectet at levels 2,6,10,14,and 18 (or 4,8,12,16, and 20)
3) A Bonus Feat can not be selected till (4th, 6th, 8th or 10th) level)
As for the Social rogue/bard combo, it's still usefull, if not different. You end up with some different abilities that way, and a lot more skill points, at a cost of a bigger HD. But the Noble I feel fits a PC arisocrat better than a bard or rogue would in general. Why should my Arisocrat have sneak attack and trap finding? Or Magical Music?