Starting QT or any other application will not affect Catalyst application in anyway you are free to do what ever you need to. Catalyst application it's works just as any other OsX Application .

Any how for what's metter I suggest you to buy a new HD where you can copy the entire OS and then modify it.

this way you can easily come back to the original disk image.

( this is not needed but it's cleaner way to manage your OS )