I have a setup with two small servers running nginx serving as proxy and a number of Apache+mod_php beefy servers. I am thinking on going over to PHP-FPM. Can I configure nginx to use a number of FastCGI servers? Or PHP-FPM itself to use workers on different machines? Or do I need to run nginx on the workers?
no you can run 1 nginx machine and many php-fpm machines.
Then in your locaction
A complement to Mike's answer.
Check this vulnerability that can arise when you have nginx and PHP-FPM in different servers. The bottom line is: don't allow users to upload content to your public directory.