IMHO the X series all came together in X3, I had X1 and hated it so I continued with 8.5. By X2 I was finding it usable. X3 is very good.

I have at least 3 times now always installed 8.5 producer as a custom install, skipping unnecessary stuff like the tutorials and additional sample content. I then install X3 and then the X3 e update. All is fine. I only install 8.5 because it includes True Piano's. Nothing else interest me, but I do think there are other items that some use. Possibly Session Drummer 2? 
I've never been told that older version corrupt the new version, The opposite seems to be true. I was told to always install the older versions first.  That the new version overwrites critical files needed for the newer version. Installing and older version over top of a new version was said to corrupt the new version?? is this correct? 
It would seem it is to me, as this has always worked. 

Be fore warned, the work flow of X3 is almost nothing like 8.5. It will take some getting use to, and relearning some things.

Some people did not like the new UI of X3. I have to admit, it took some getting use to. But in the end my.work flow is much better than it ever was.

Johnny you are correct. If you are going to install older versions of Sonar along with new versions ( such as setting up a new PC), I stall older versions first.

If your going to do a custom install of some plugs from an older version, then you can do it no matter what version you have on the PC.

But if you do a custom install of a newer version, leaving out the plugs you already have installed, and then uninstall the older version later, you will uninstall certain plugs that are tied to the version of Sonar. Thus making them unavailable to the newer version and any projects that need them.