I just used the festival plaza with or without wifi (not sure which)
Im just wondering if there is a way to transfer pokemon from heart gold to the new games, too. Its a HG cart.
Then you didn't make the same mistake as some of these fools, so you have no probs using an illegitimate copy of Sun now.
As for HG, your best bet is to extract save and use PKhex to inject pokemon to Sun/Moon. May have to edit them a bit to appear to have been brought up gradually (HG -> Black -> XYORAS), but I will let you dig up this research. (Reverse engineering this is half the fun!)
To extract save from ds games use this homebrew app in 3DS: https://gbatemp.net/threads/release-twlsavetool-v1-0.412887/