Fortunately I did this into a testing environment.
I installed Debian 10 then I installed Percona Server 5.7. I ran Virtualmin install script and it crashed on installation Phase 2 "Installing virtualmin-lamp-stack ". Checking the errors the script crashed removing Percona packages installed before. I had to open a Terminal to uninstall all Percona traces then I ran again the installation script. This time there were no issues. Percona was vanished and MariaDB replaced it.
Why any MySQL server installed before is removed by Virtualmin script and it is replaced with MariaDB? I know MariaDB is the default MySQL Server in Buster but I prefer Percona. If you remember in the past with Debian 7 and 8 we could install any MySQL server before without getting any issues at Virtualmin installation. The script was skipping MySQL server installation if a version already was available.