Thread locking fluid on fixed cog
If you have a fixed gear type hub, not a dual freewheel flip flop hub, and a lockring which is properly installed, there is no benefit to using threadlock on a fixie hub. 

Thread lock works just fine on aluminum, although it tends to cause stripped fasteners because it requires more torque than most aluminum fasteners can handle. With a fixie lockring, though, there aren't enough threads (surface area) to really benefit from a threadlocker.