Ansys Employee

Fluent Meshing always uses tri mesh when doing the initial surface meshing. When you mesh the volume it remeshes using poly cells. If you have tri cells at the end of the fault tolerant workflow you may have missed a setting. 

No reason why you can't use watertight with impellers with mrf zones. You just need to be careful with the zone definitions. If you want sliding mesh, you could build two models. Look up "append mesh" in Fluent.