There's a rule about this for summoned creatures. The "Spell Description" portion of the SRD says this about Summoned creatures:
This is true for Summoned creatures only. Called creatures, or creatures you encounter normally, don't have that.A summoned creature cannot use any innate summoning abilities it may have, and it refuses to cast any spells that would cost it XP, or to use any spell-like abilities that would cost XP if they were spells.
There is a line in the "Monsters and Class Levels" section that might apply.
Monsters And Class Levels
If a creature acquires a character class, it follows the rules for multiclass characters.
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Level Adjustment and Effective Character Level
To determine the effective character level (ECL) of a monster character, add its level adjustment to its racial Hit Dice and character class levels. The monster is considered to have experience points equal to the minimum needed to be a character of its ECL.