If I could place the RX100 sensor in A500 for bird shooting, .........

that if I use a lens that was designed for a full-frame camera, like the A99, on an APS-C sized senor camera like the A57, that the aperture is smaller? Thus, a lens with a f/2.8, is af/4.2, and the 500mm f/8, which is a FF lens, would have an aperture of f/12? Funny, because the AF system won't work with such a small aperture.

If what you say is true, Sony can expect to be sued any day now.