Is it permissible to wash a soldier’s uniform during the 9 days? And if yes, is it ok to add other non-related items to fill up the washing-machine. Also, are underclothes part of the washing ban?

One may laundry a soldier's uniform if it is uncomfortable to walk in them as they are. It is forbidden to add to a permitted laundry other clothes. All clothes are under the washing ban including underwear. Rabbi Ro'i Margalit