I had a bad experience when I accidently opened a workspace configured in RAD 7 in RAD 6. It messed up some project files.
For running servers on both RAD 6 and 7 together, you may need to iron out port dependencies. I had different http ports configured for RAD 6 and RAD 7. Still it did not work for me, cause it uses few other ports for communication during publish.
If you have a work around using which you got both running together please let me know.