I didnt know that was a Tag, Max.
Anyway, there are some things Abe has possessed in the past which dont have souls, so I dont think possession is really based on the having of a soul, more on the possessers choice. Who would want to possess a Slurg or a Fleech anyway? I imagine it would be quite uncomfortable for Abe, and probably difficult to control too as they arent the smartest creatures on Oddworld. In most cases, the primitive mind can overcome the dominant/intelligent mind. Has anyone ever seen Animorphs or read the books?