Some creatures explicitly cast spells as an Nth-level $class, such as Nymphs. What happens if such a creature takes class levels in that class? Do the class levels stack, are separate spell lists generated as though a different spell casting class had been taken, or what?
Unless otherwise noted, you can assume racial casting levels stack. You'll find the rule to that effect, among other places, in the section on monster advancement here:
If the creature possesses class features (such as spellcasting or sneak attack) for the class that is being added, these abilities stack.
It's worth noting that this only applies for spellcasting; spell-like abilities are, unless otherwise noted, cast at a set level. If no caster level is given, then you can refer to the general rules on spell-like abilities, which defaults to the class level they are gained at. For monsters, the default caster level is instead equal to their hit dice.