MariaDB 5.5 -- how to customize password rules
A couple of comments:


Re: "the standard linux-user-name-matches-the-mysql-account-name authentication" - I assume you mean using the unix_socket plugin for authentication. This only works on localhost, i.e. the user has to be logged in on the database server.
There is no MariaDB 5.7, but there is however a MySQL 5.7.
There is no need to build your own RPM for MariaDB 10.x on RHEL7. You can just add the relevant MariaDB 10.x .repo file to your /etc/yum.repos.d/ directory. You can use the repo config tool on mariadb.org.


It's up to you to decide whether it's worth the effort to upgrade to MariaDB 10.1+ and thereby be able to use the password checking plugins - simple_password_check and cracklib_password_check. (10.4 is now GA, though at the moment this is still relatively new, so you may want to consider 10.3 or older as a more stable release.)