Within AE, Twixtor can work directly from the original frame rate.
You can just drop the footage in the 24P timeline (comp settings 23.976) and set the Input Frame Rate slider in Twixtor to 59.94. If all you were doing is a frame rate conversion 60P to 24P, that is all you have to do.
You can also interpret the footage to 23.976 and in Twixtor then set Input Frame Rate then to 23.976. That will also work, that said, if you drop 59.94 into a 23.976 FPS comp.
To achieve the equivalent of “as if it was shot at 1000 FPS”, then even if you are in a 24P comp, the formula remains 59.94/1000.0 = a bit over 0.05 % for speed for “as if it was shot at 1000 FPS”. The 24P comp is only then defining the playback rate for the slowmo. That is as explained here, if you were in a 30P comp, Twixtor would generate more frames per second, and different ones but say at 5 seconds on the timeline the “slowmoed” action would be at the same place.
check tutorials for twixtor here at the cow or at help.revisionfx.com