Before you add your stills, go to Options/Preferences/Editing tab and select the amount of time there. I don't think you can adjust by frames automatically. It might depend on what chronological format you have your timeline setup for. FYI - Setting your still length to just a frame may be too quick for anyone to see. This may be good for subliminal messages.
You can do what you describe rather easily. First of make sure all your pictures are in numerical order.1-500. Open vegas, new project, then open, a pop up window will open for your project settings select ok. Then another pop-up will follow asking you to select what you want to open. Navigate to your #1 picture select it, then the bottom left of the same window make sure to check "Open Still image sequence" Then in the range box do 1-500. After that all the pictures will be added to the timeline at once,one per frame as you requested.