Sorry to necro, but this applies to a character I'm currently running.
I don't see why it wouldn't work with an improved familiar. I thought pathfinder had a clause that "Specific trumps general" when it came to rules. The general rule for "Beast Shape #" is: "When you cast this spell, you can assume the form of any Small or Medium creature of the animal type"
The witch's Familiar Form is a specific, that allows her to take the form of her familiar as if using beast shape II.
As in, this specific rule allows the familiar to be a qualifying shape, trumping the general rule.
So, it seems reasonable that even if her familiar doesn't fit the "animal" type, that it would be fine. There might not be various sizes of the forms, as Edgar quickly pointed out, but it should still work.