Do the caster levels from a Prestige Class stack with the Arcane Class (Wizard/Sorcerer) levels for determining what you can select as an Improved Familiar?
The Wizard Familiar specifically states "Wizard" level, which to me suggests that a Mystic Theurge or Arcane Trickster (for example) would not have their familiar improve further despite higher effective caster levels (though hit points would still improve).
However, the Core Rules state this on Improved Familiar: "Arcane Spellcaster Level" - would a level of a Prestige Class with arcane spellcasting attributes (such as the caster level) also stack for what type of Improved Familiar you can get?
Any ideas on if Prestige Class levels stack with Wizard or Sorcerer levels on what level of Improved Familiar you can get?
Seems legit. The caster levels from a prestige class improve your spellcaster level, so you could choose a higher level familiar based on your spellcasting level. Note that your familiar's abilities are still only based on classes that grant familiars, so if you only have 5 levels of wizard and 2 levels of a arcane trickster, your familiar would have abilities of a 5th level wizard as the arcane trickster does not have a familiar class ability.
EDIT: Ninja'd. Only stacks if all classes have familiars.
The easiest solution is to have an arcane SLA. That gives you an arcane spellcaster level equal to your level and your set.