Jules,
I have used the Pyrodex pellets (2 x 50 grain) with the 435 grain White Shooting Star Sabots in my White G-Series Whitetail with #11 caps. They lit the Pyrodex pellets fine, and I had awesome accuracy with the above load. I doubt that they would be reliable with 777 Pellets, but I never tried either.
The Pyrodex Pellets have a layer of real Black Powder bonded to one end to aid in ignition, 777 pellets don't.
I never tried more than two pellets, even though my .504 White is rated for 150 grains of loose powder.