Confusion! Please help.

Hi everyone! 😊 Today's episode was nice but I just have a confusion. Wasn't there a rule that any particular warrior will fight against another warrior who uses the same weapon as his. Then why did they show Ashwatthama fighting with a Panchaliputra (don't remember his name but I guess it was Yudi-Draupadi son) as Ashwatthama was using a bow whereas the Panchaliputra was using a spear moreover according to the rules of the war the warriors must also be on the same mount. Well leave Ashwatthama even Mahamahim Bhishma attacked Yudi with a bow while he was using a spear to fight . So, wasn't he doing adharma?
Please help I'm utterly confused.