FlightSafety--
Would seem to me that:
One dedicated crew for the 757....
One dedicated crew for the GV...(or GIVSP)
plus another Capt for the GV...
The third Captain would be the "swing guy" Captain or First officer, as the need indicates.
Noted last year in SNN a GV parked on the ramp, and the handling agent mentioned that the OWNER was beside himself because...the number one engine had seized at 30 West...on a brand new aircraft.
Think the GIV-SP would be your best bet, the GV has MANY problems (according to NBAA, and I am a member).
The Falcon is a good aircraft but, if you ever have to buy parts, prepare to pay BIG TIME.