Inventor to Revit is fairly simple, save as an .rfa file type inside of inventor, which is translated as a family in revit.
What exactly are you trying to accomplish going from plant 3d to Revit? If you're doing your Arch, structural in revit, and your MEP in plant 3d, I'd keep the models separate. Just link your plant 3d model into Revit, or export you revit model as a DWG and Xref it in plant 3d. If you merge the two models you'll lose the intelligence, which defeats the purpose of using BIM. Whether you're losing intelligence from piping, or from your revit model.
Hope it was helpful
Mike
