- Is the system enough to run Proxmox + OMV virtualized whith good performance? And another server OS (like ubuntu server) just with the idea to run docker or small app like transmission indepently?
Yes it's enough. You can even create an LXC container with docker in it instead of a full VM.
- Is it possible to mount my RAID1 data disk on OMV? I see that is possible to "pass" the disk to a VM, but the OMV VM will be capable to "see" the RAID without loss of data? Any other solution (without build a new array)?
Proxmox can see your disks "as is" and you can pass-through them to your OMV VM . After that you need to remount the RAID , if you know how to do it. I think it can work but not sure, I don't use raid.
You can also add an HBA card. Proxmox can pass-through the card and your disks will be directly managed by your VM and proxmox will not see them anymore. Pretty sure this solution will work.