It's allowed (and you can go to Deansgate G-Mex as well if the urge strikes you), although as 185 said, if you get a bad G4S Northern clerk (which is most of them) you may have to argue the point with them.
If you get the xx:20 departure from Euston to Manchester it's easier to change at Stockport to the Liverpool train, which will be on the same or the adjacent platform at Stockport opposed to changing at Piccadilly.
If you have a ticket to Manchester Stations does it include Old Trafford/Manchester United's ground?
No. Manchester STNS includes Piccadilly, Victoria, Oxford Road and Deansgate. For Manchester United you need to buy an extra ticket costing £3.50
Or possibly less, if an overdistance excess is available.
Man United's ground is also excluded from TfGM zonal tickets.
We have zonal tickets now?
You mean the System One Travelcards sponsered by TfGM?