Cubase backwards compatible license ???

Can I run Cubase 6 if I have a Cubase 9 license?

yes

Depends on what exact version the “Cubase 9” License is.

I’m talking about a normal Cubase 9 Pro license.

My Cubase 9 Pro license can run Cubase 7~9(I only tried those three version),
I thought because I brought those three before, so actually I can run all Cubase version before Cubase 9?

Then, as Jaslan said: Yes.