There is unfortunately no way to mix symmetry with non-symmetry in rosetta. The binding of a protein to the hexamer makes is non-symmetric – unless a protein is bound to every subunit in the hexamer. So you are stuck with non-symmetric docking and all atom refinement. If you want to maintain the symmetry of the hexamer during the refinement you would have to add some sort of contstraint. But it would probably not help much with speed.