Assuming the NPC species/type/subtype would be a valid choice for both the cohort and familiar features.
For the sake of a complete example, lets take a Coure Eladrin.
Wizard is CG, has the exalted familiar feat, and is lv 20 (Can have a coure eladrin as familiar).
Wizard has the leadership feat, and over 25 leadership score (even though 25 is the cap). Has no cohort at the moment.
Wizard currently has no familiar, and is not on "familiar replacement cooldown".
Wizard has access to the eladrin homeland in Arborea, and several coure are willing to go into wizard's service. Permission from the higher ups is also assumed.
Can the wizard take a Coure with class levels (within the ECL allowed by the Leadership feat) and make that Coure the wizard's familiar?
Or does the wizard must walk out with two coures, one with class levels and another with the basic outsider 2 HD?
The DM is indifferent on the subject but would prefer to stick to the written rules. What do the books say?