Do yourself a favor.
Do the default install, and yes it will put the P9 Runtime in "my documents".
Thats good.

Poser9 will create its own runtime to work in c/program files. Do not touch that one.

Your P9 content will be in your "My docuents" folder.
And yes you can link to your P7 runtime, no problems there.

Poser 1, 2, 3, 4, 5, 7, P8 and PPro2010, P9 and PP2012, P10 and PP2014 Game Dev
"Do not drive faster then your angel can fly"!