> partition the system using 2 partitions. / (for the system, /data /serv or /srv for storing user information.
This will not work as it will cause apache suexec to fail. The suexec directory is compiled into apache. So if you want to have a separate partition for user data, then you will have to use /var or /var/www and /var/vmail. Only For OpenSuSE the directory /srv can be used. A directory /data will not work on any Linux distribution.
Also your proposed layout is incompatible with ispconfig. In ISPConfig, every website has its own user. The layout required for ispconfig is:
The layout that ispconfig currently uses has been discussed by the developers before the ispconfig 3 development started and is the result of experiences which were made with ispconfig 2 and several other controlpanels to match the requirtements for hosting campanies.