I've never needed heatsinks on any type of track, including Atlas plastic tie track. In truth, any clip-on heatsink on the track is not going to make a bit of difference as far as melting ties on Atlas track, the track is a far better heatsink than any clip-on will be.
We use Atlas track on our modular club modules, and I've soldered hundreds of drops on them with my Weller gun, never an issue melting the ties. Get in quick, get out quick and there's no issue. It's imperative that you clean the spot on the track for the solder joint, I use the Dremel with a fiber cutoff wheel at slow speed to scuff off the surface and expose a clean spot for soldering. Plain 63/37 Rosin Core solder is all I've ever used, and my can of flux is in the drawer for work on brass locomotives, I've never needed it for track.