Catalyst 16.9+ drivers do not allow 6 GPUs in Win7 even with 6xGPU patch. Try Win10.

To troubleshoot ubuntu it is required some more information.

1. /var/log/syslog
2. /var/log/Xorg.log or smth similiar

have you added your user to video group?

I use ubuntu 16.04 server on my rigs with no bugs. Try it maybe