I really don't understand your point here. Gandalf is present in the source material for this event. He gives Aragorn the quest to capture Gollum and then later on interrogates a captured Gollum in Thranduil's Halls in Mirkwood.

This would mean that characters like Gandalf, Aragorn, Legolas and Thranduil are perfectly fine to appear. As as their actors. This is a no-issue honestly.