So basically I can just take a header from any online game and apply to whatever .3ds rom I want and the online will work right?
Well some are apparently incompatible, example the 128mb file header from Pilotwings may not work on all games, it's best to use one 4gb size from a game like:
(Resident evil revelations, Dk country returns 3d, MGS: SNake eater 3d, MK7)
If you know the game it's better cus 0% chance of being banned, slim chance of being banned with online ones but still, I'd use MK7, it should work fine and then rename it .3dz