OK, so there's 5 SPI devices and it looks like there's no hardware conflicts:
Summary:
[b]CS Pin Name Board[/b]
D53 SS Mega SPI master/salve (set as output for master) (reply#6)
D8 GPU SEL Display
D9 SD SEL Display
D4 SD SS Wifi Shield
D10 Wifi Wifi Shield
EDIT: Reply#7 is the correct method for CS control.