Most of my cookware is aluminum. I've seen induction interface disks, but they are quite expensive. Would it work to use a small cast iron griddle as an interface disk for my aluminum pots and pans?
Yes, it will work. The induction interface disks are practically the same thing as a griddle.
Of course, both disks and griddles mean that the induction stove will behave like a resistive stove (slow heating, etc.) If you want to get your money's worth from the induction stove, replace your cookware. The combination with cast iron is great, as it gives you all the advantages of cast iron without the major disadvantage of glacially slow heating of thick pans. All other induction-capable materials will work better than alu on a griddle/disk too.