the use of the H8000FW and a FireWire audio interface is possible *IF* the DAW software allows drivers aggregation. Logic and ProTools so support this. Otherwise you are restricted to a single FW driver. Hopefully software houses will enable this on all DAW out there.
The advantages of using the Eventide on FW would be the ability to send/return audio from your tracks for external fx processing, all in digital domain.
Regarding vocal harmonies, what exactly do you need to do?
Pitch shifting comes in 2 major flavours, chromatic and diatonic.
Chromatic: pitch is shifted in 1 cent steps. 100 cents equal a musical semitone (half step).
Diatonic: this variety takes into consideration tonal music system parameters such as Key, Scale and Interval. You set them to the song key and scale and pitch shifting will stay within that scale.
all the best