what are the possibilities if I need to incorporate Starling inside an existing Project that used the traditional Flash DisplayList?
I know that the GPU rendered items are always "on the bottom" but can I also position and move the Starling viewport to a defined area?
Starling's viewPort is a Rectangle with x, y, width, and height properties, and it is not required to fill the entire screen. You can position it anywhere.
I noticed that it did not really work with
var scaler = new ScreenDensityScaleFactorManager( starling );
but maybe i got something else wrong.
That is correct. ScreenDensityScaleFactorManager is designed to completely control the viewPort and stageWidth/stageHeight. If you want to handle the viewPort yourself, you cannot use ScreenDensityScaleFactorManager and will need to implement your own custom scaling behavior.