Will I damage anything if I connect Uno to PC with USB cable (that also has 5V supply)?
QuoteWill I damage anything if I connect Uno to PC with USB cable (that also has 5V supply)?No. The Arduino has a power selector circuit that handles two separate power inputs. When USB power is present, that is used. When it isn't the Vin pin power is used.
One way to eliminate this possible problem is to make sure that the +5vdc power on your shield board does not connect to the +5vdc shield connector power pin. That would keep the two regulators isolated from each other. But then he would have the problem of his shield would not have any +5vdc power if he was running on USB power. Oh the evil webs we weave.
Let us know if it lets any of the magic smoke out.