How to set default php handler to FPM server-wide or for service plan?
It is not available in Service Plan.
This is a software issue with internal ID PPPM-3326. It is already fixed in Plesk 12.5 MU #4.
Default PHP handler for Service plan can be also set using CLI:
# /usr/local/psa/bin/service_plan --update 'Service_plan_name' -php_handler_id plesk-php56-fpm -php_served_by_nginx true