Yea man it's fine, i do it every day. I even made a Wikipedia article listing all the games with download play because there wasn't a good one anywhere else. Have fun! If you find any not on the wiki page add em' in http://en.m.wikipedia.org/wiki/List_of_Nintendo_3DS_Local_Multiplayer_gamesI only use roms for games. Cias only for themes and apps like bigblue menu etc
So if I launch lets say ssb on old 3ds and try to play multiplayer locally with my n3ds I wont have a problem. right?