The Inventor Routed Systems package does contain a large library for fittings and pipes. It is fully customizable and is based on the Content Center. It uses a really nice 3D routing tool to create the run and then it populates it with the fittings.
Inventor will not use an AutoCAD drawing as an X-REF, but you can overlay it in the model. The new Inventor Factory coming to Labs shortly may have this functionality but I can't say any more about it yet.
Inventor Views can be placed into AutoCAD from the ADCENTER though. So if you create a piece of machinery and want to see it in a layout in a factory you can quickly place the view you created in an Inventor drawing into AutoCAD by dragging and dropping.
As a monetary decision, you do also get AutoCAD with Inventor so this could reduce your subscription costs overall.