The Mad Linguist totally ninja'd, but I'll explain in depth.
Unfortunately I think SkyDragonKnight was on the right track.
I went ahead and 'fixed' this since you added it to the dirty trick handbook, but I'm having doubts. I was thinking... 'there is no way the language of the ability didn't something along the lines of my "fix."' It appears I was right:
the familiar can change its form to that of any creature that the wizard could normally have as a familiar.
This "normal" seems very important. It seems to me that only animals that have a familiar entry could be considered normal to have a familiar. This is close but not quite. Im really not that much of a stickler (honestly) so long as the RAI supports your interpretations... but in this case it doesn't.
I'm sorry I don't think this works (perhaps you can salvage it by digging up
all the improved familiar forms and finding a way to make them considered 'normal familiars' without requiring the improved familiar feat a bunch of times)