Obviously people duel in a variety of mods and its impractical to have tons of duels servers up of various mods and TC amounts to satisfy everyone so I think ABD and maybe boxshu should be fine seeing as they are overwhelmingly the two most popular duel mods.
The main point is just to provide a central hub to find duels almost and stop people from having to just wondering around looking for people who want to duel, not to end all need for private duel rooms.