ABOUT SECURITY ! ! ! ! ! !
No one mentioned, but putting all sites together is 10x increased risk to your business! In case 1 (from all 6 sites) is hacked, then all your other sites are hacked too!
I don't know if there is a complete solution to separate domains from each-other COMPLETELY. It's not enough to restrict only sFTP user. Instead, there should be access-jails (which will restrict APACHE PHP process) to have access to upper directory files in server.
I am trying to find out if there is any way.