Is the Polaroid 500 instant film compatible with Polaroid 600 instant film?

Sorry, but you really can't make that work. Since Polaroid is going out of the film business, you might consider getting a cheap type 600 camera from a thrift store, and use it until you can't find any more film.

Your other options are to get a packfilm camera, as Fuji continues to make the 3-1/4 x 4-1/4 packfilm.

For compatible cameras, check out the Land List:

Or you can get a Fuji Instax instant film camera, Fuji's own brand of instant photographhy.

I found some 500 film on Amazon, but I don't know if you can expect to get non-expired film from there reliably. 600 sheets are ever so slightly bigger, so they won't fit properly into a camera that uses 500.

It looks like they have it at Best Buy!