If you want to use multipe (older) versions off Citrix XenCenter on one Windows server, you have to install the application in this order:

1. start with the oldest version - e.g. 6.50
2. use the default directory (c:\Program Files (x86)\Citrix\XenCenter)
3. if the installation ends without errors, rename the directory to the version number (e.g. XenCenter650)
4. start with the nex version - e.g. 7.12
5. use the default directory again
6. rename this dir to XenCenter712 again
7. install e.g. 8.20
8. and rename this also …

Now you have three working version.

This is needed, because the old XenServer 6.50 server and pools can't managed by the new versions of XenCenter.