Yes, yes, and yes.
You can add new file types for Gallery to recognize. For example, a default installation already has MP3 added so you'd just have to go in add RST as a valid file extension.
For the graphics images are displayed as thumbnails but non-image files (like MP3, video clips, zip files, etc.) are displayed as a generic icon. The generic icons are stored in your ../gallery/images folder. In your case after you added RST as a valid extension you'd then add a "../gallery/images/file_rst.gif" file and then that image will be displayed for any RST files that are uploaded.
EDIT for #3: Yes, files in the gallery can be both rated and have comments left for them.
But... if the only thing you're planning on using the Gallery for is RST and MP3 then it may be an overkill for your needs. Are planning on eventually offering images as well or just audio files?