I'm using a CloudLinux server, and I want my domain to use an ALT-PHP version. Should I set my domain to use an ALT-PHP version via "WHM / Software / MultiPHP Manager?"
You should not configure any domains to use an ALT-PHP version via "WHM / Software / MultiPHP Manager."
The default PHP extensions on ALT-PHP versions are limited, so doing this will result in unavailable PHP extensions. By default, the PHP modules and extensions within /opt/alt/php*/etc/php.d/default.ini are commented out, which is the reason they aren't loaded in the default ALT-PHP versions.
The best way to configure a domain to use an ALT-PHP version would be to set the domain to "inherit" via "WHM / Software / MultiPHP Manager," and then specify the ALT-PHP version within "cPanel / Software / Select PHP Version."