Hello! The short answer is yes, the alien worm can absolutely stay if you brought him into the game !

You're right in thinking any benefits to Konstantin's little friend inhabiting would be nerfed, yes. However any negatives would still be in play if the player wishes to keep them, such as the need for blood. They'd be stuck together for better or for worse, blessed with suck.

Since I'm not canon familiar, if the entity is something that comes out to feed, this would be nerfed and it would be Konstantin himself who needs to do the blood drinking in order to provide the sustenance it needs. Feeding from animals is fine, but I would ask that any feeding from humans would be discussed OOCly so that everyone's on the same page.