When I do my "MegaMan Hard Mode," I disallow all use of Special Weapons (Items/Rush included), Hearts (X games), Sub Tanks/ E Tanks, and Armors (Including MM6 Rush equips). However, like Creamer said, there are times where your pathway is impassible without an item, armor or special weapon. In that case, use of them is acceptable only at that instance. Once your path is passable without the use of the item, weapon, or armor, then it's use is disallowed.
Also, I'm all for going in order. Like if the challenge is simplistic and systematic like this, we can just submit this challenge for all the games and let people play at their leasure.
For a rule:
1. Any use of items or special weapons besides the Mega-Buster/X-Buster, is disallowed. However, if you pathway is completely impassible, use of the a required item is acceptable until impassible task is complete.