If this shoudl ever come up in my game.
I might allow familiars to get levels BUT
it would be under the following rules.
The familiar needs to gain abilities and such from their master. Hence if the master is level 10, I'd allow that if he has enough experience to gain level 11, he may opt to put it to the familiar instead, loosing all those exp to the bottom of 10 again while the familiar gains that 1 level.
The familiar can never gain more levels than 1/4 of the wizard. IE, level 20 wizard could have at the most a level 5 familiar in addition to all the other powers the familiar already has.