I have found a code in Mega Man 8 that allows you to access second set of Robot Masters without beating the first one (US Saturn version, emulator only).
To be specific, a hex edit and its location (second screenshot, click on it to see it fully) that allows you to switch between rosters from the get-go, allowing for more experimentation with weapons and boss orders. Couple of notes:
-MOST IMPORTANT ONE!! - After applying the hex edit (the changed numbers highlighted in blue and pink in the second screenshot), the change button at stage select won't work emidetely. You will be able to highlight it but not press it. You will have to enter and leave the shop first and after that it will work just fine
-It is impossible to beat Sword Man stage without Thunder Claw and Tornado Hold, but you don't need Ice Wave and Flash Bomb
-It is also imposible to beat Search Man stage without Thunder Claw, unless you're a speedrunner-level Mega Baller and able to perform that tight Rush Bike jump in the spike section
-You won't able to fight Duo with this code on
-It is possible to enter Wily stages without beating Frost Man and Grenade Man.
If you don't know how to apply it or how hex editing works, let me know in the comments. Have fun!