"Is the HO alternator just needed for the Taurus fan setup, or would you need a HO alternator for any of the factory fans?"
That all depends on the rest of your electic loads.
Fans main draw is at start up (good spot to use a capacitor) where they double thier draw until they are up to rpm. I have a set-up from a FWD GM car and they spike at 40amps then settle to 20amp.
I would wire a dual fan set-up with 2 different temp sensors ,say 160 and the other at 180 degrees. They would stagger start and there would be little "dual" run time unless sitting in traffic in Texas with the air on.
You could also wire a sensor into the trans cooler to keep the temp down. It would also help cool the engine as it draws threw the rad.
I plan on changing my race car to just this set-up. Had to do quikie wiring to get to the track so they are on seperate toggles/relays for now.
The car will cool in staging lanes with both fans on.
Here is the proposed wiring diagram, made is MS Paint.