"I thought the Pi midhorn was made to use the 10" Alfa driver. Wouldn't a 5" driver be too small for the horn throat?"

I am not sure if the Pi mid-horn is specific to the Alpha 10".
I belive that Wayne has used a JBL 10" driver with this horn as well.
Possibly he will chime in here and fill us in on other driver options?

About the 5" Faital driver, the guy who posted using it in the tractrix horn used it with a throat area that was round and close to the diameter of the driver. I honestly don't have a clue about the physical requirements to use such a driver. I just thought I would throw it out there as a possible alternative.
For what it is worth, the guy who used the 5" Faital driver replaced an 8" B&C, claiming that it was as dynamic and could keep up with his system.

Good luck with your project, and have fun.

NW