2rail rolling stock

Yes, I have run it for years.

will atlas 2-rail rolling stock run on atlas 21st century 3-rail track?

They will roll on the rails just fine. The problem comes at switches and crossings.  Three rail wheels are designed to ride through frogs on the flange.  Low flanged RP-25 (2 rail) wheels will dip and bounce through three rail frogs.  Derailments will not happen every time but are much more likely than when running three rail wheels through three rail switches and crossings or when running 2 rail wheels through 2 rail switches and crossings.

The same goes for Gargraves/Ross or MTH ScaleTrax.

I've been running scale-wheeled rolling stock from Atlas, Weaver, Pecos River Brass and Ajin pulled by MTH scale-wheeled locomotives on the club 3-rail layout (Atlas and Gargraves track with Ross turnouts) since 2009. Our track is good for the most part but we do have some bad spots. I've had a few Atlas cars that had a minor misalignment in the trucks that caused them to derail, but over 90% of them have had no issues with our turnouts, even an O-72 on the run-around track in the yard. Of those few cars, the problem was resolved by removing the cross-bar on the truck which allowed the truck side frames to swivel slightly (equalization). The mainline turnouts are 11-degree (#5) and they work just fine. I have several YouTube videos of testing runs (trains ran all day). See the link in my signature.