the whole system will rotate

Please keep the list in the loop.

There is no difference i this respect between
fixed and shrink-wrap boundaries. If you start
with no angular momentum and run NVE or NVT
(not Langevin) and have no external forces,
I don't see how the system can rotate, since
no angular momentum is added. Fix langevin
might add some noise to the system. Assuming
your dynamics are good (small enough timestep).