[quote=@Major Ursa]
[@Lugia]No, I mean like how the souls would balance out with the body. Monster souls don't usually have bodies full of so much matter, while human souls aren't usually housed in a body containing monster magic...while still having their determination. I suppose the souls could still be tethered to the body in some way, as they keep in close proximity with the group as a whole, but once they stray the bodies might be forced to adapt to the different soul. I mean, isn't an amalgamation a monster with high amounts of human determination?

*situation intensifies*
[/quote]

You're overlooking something, my ferret friend: The spell itself. The original casters just want to swap the bodies of their targets, not kill through torturous means. So, their spell would have protective measures to keep the balance between soul and body just in case there was a monster and human swap.