Stacking Caster Levels with Monster Casting

[VoxRationis]

If a character starts with a monster that has spellcasting as part of its racial abilities, and takes levels in the same spellcasting class that it naturally casts, do the levels stack?
That is, if something has 6th-level sorcerer casting natively and takes another level in sorcerer, does it have casting Sor 6/Sor1 or Sor 7?

[Karnith]

If a character starts with a monster that has spellcasting as part of its racial abilities, and takes levels in the same spellcasting class that it naturally casts, do the levels stack?
Yes, they stack. See e.g. here in the SRD (http://www.d20srd.org/srd/improvingMonsters.htm#associatedClassLevels):

A spellcasting class is an associated class for a creature that already has the ability to cast spells as a character of the class in question, since the monster’s levels in the spellcasting class stack with its innate spellcasting ability.
Many monster descriptions also include reminders of this rule. See e.g. the Rakshasa entry on the SRD (http://www.d20srd.org/srd/monsters/rakshasa.htm#rakshasasAsCharacters):

Spells: A rakshasa character casts spells as a 7th-level sorcerer. If the character takes additional levels of sorcerer, these levels stack with the rakshasa’s base spellcasting ability for spells known, spells per day, and other effects dependent on caster level. A rakshasa character likewise uses the sum of its racial spellcasting levels and class levels to determine the abilities of its familiar.

[Urpriest]

Yes, they stack. See e.g. here in the SRD (http://www.d20srd.org/srd/improvingMonsters.htm#associatedClassLevels):

Many monster descriptions also include reminders of this rule. See e.g. the Rakshasa entry on the SRD (http://www.d20srd.org/srd/monsters/rakshasa.htm#rakshasasAsCharacters):

I'd never noticed before that Rakshasa stacks for Familiar advancement too. That's an interesting benefit.