Question is quite simple, as I do not yet get the new multiplayer setup options. Is the Dark Tribe mechanic present in mutliplayer? Is it map depended? Can I have the Dark Tribe on a random map?

afaik there is no dark tribe in mutliplayer matches.

The dark tribe was left out of the multiplayer game because of the way it works... No own land, being able to"steal"peoples ressources and territory, etc.

HOWEVER, some people did find a way to play as the Dark Tribe on custom made maps using some exploits within the map editor...
(Here's a link to an example map, it's SP though:
http://www.siedler-maps.de/s4maps/StrangeAlliance-7063.htm?sid=e93f92676041b52d11b501a528c9d618 )

So, theoretically it IS possible to do so... I mean playing as them. But only on custom made maps. There's no dark tribe on random maps, at all.

Hope I could help!