I don't think that's the intent of Kitsune Change Shape. It states that you always turn into the same specific human. There is no reason that a kitsune who can turn into a thin female human would be better at impersonating a fat male human than another fat male human would be, all other things equal. I believe the intent is +10 to pass as human - to hide the fact that you're a Kitsune - which shouldn't apply if you're trying to fool people familiar with your specific human form.
The reference to Alter Self definitely prevents disguising yourself as a specific human, since you can't (barring Realistic Likeness) use a polymorph effect like Alter Self to turn into a specific individual.
I would eliminate the -2 penalty for disguising yourself as a different race for a kitsune in human form disguising themselves as a different human, but that's just me.