Oh, that? Well, that's a good question. They're different from regular ghosts so I suppose they just manifested in a way that was easy for them to share a space.

[Flora, Feather, and Fang were rather unique, more akin to nature spirits or something akin to it. They were mysterious even to other espers and magickind.]

So to circle back, I wouldn't be too worried about poltergeists. Fang tends to scare them off the most.