In the Server Manager, there's a PGSQL module, but is there one available for MySQL? I don't know who in their right mind would run both at the same time, and I don't know why Plesk didn't write this module to be configurable between the different types of SQL servers.

Does anyone know of a module that can replace the PGSQL manager with MysQL?

There's no need for one, MySQL was configured the moment Plesk was installed. The module you speak of is to set the master passwd for pgsql, once thats set you can create pgsql databases from plesk. Until you set that option you can only set up MySQL databases from plesk.