So it doesn't have to be the same game? Just any 3DS game will do?
Also do I have to do this with every .3dz rom I want to go online with or is this a special case?
Any 3ds game will work (preferable one you bought new and will never sell!)
Pokemon ORASXY, Super Smash Bros, Bravely Default, and Tomodatchi Life will get you banned if you don't use a private header
I use a private header from skylanders giants and that game doesn't even have online features.