You probably want to be using mongrel::cluster with the --clean flag.
Mongrel_rails won't restart daemons that were hard killed by Monit and
didn't remove their PID file.
I started by using /usr/local/bin/mongrel_rails cluster:: but that
calls mongrel_rails (without an absolute path) which monit can't find.
Monit has a very slim PATH setup and you can't add to it. I guess I
could add a symlink from a place in the Monit PATH but I have a hard
time believing that will fix the problem since my start and stop
programs work fine from the command line.