Ae is not asking to change fps to match composition settings

If you want to slow down the 500 FPS footage to 25 fps, select the footage in the Project Panel, use the Layer/Interpret Footage menu (or right-click), and set the frame rate to 25 fps. This will slow down the footage so that one second of real-time will take 20 seconds of screen time. 

There is no automated way to do that. The composition's frame rate has nothing to do with the speed of the footage. Footage interpretation is the only thing that controls the speed of playback. Matching a composition's frame rate to the footage means that each frame of the footage will match each frame of the composition.

AE doesn't ask you this, only Premiere does. To create a comp matching your clip's settings, drag your footage to the Composition icon at the bottom-right of the Project panel or right-click and choose "New Comp From Selection."