I think it should be that any pokemon that can qualify into the random matchup battles should be able to battle. Including ubers. But just not promos as to be nice to people like mew. Just to let you know, for us in America, the event wasn't wifi. We had to go to the GameStop in the town 3 towns away and then walk through a mall with a bulging original DS.
Sides, the list of what can't be used in random matchup is right here:
Mewtwo, mew, Lugia, ho-oh, celebi, Kyogre, groudon, rayquaza, Jirachi, deoxys, chatot, Dialga, palkia, giatina, Phione, manaphy, darkrai, Shaymin arceus, victini, Reshiram, Zekrom, Kyurem, Keldo, Meloetta, Genosect.
And for some reason, Chatot is banned too.
Copypastaed directly from source.