I cannot tell you how many times we have tried to split the server into more than one game. People are an 8v8 playing DSD, so we try and break off for a small 2v2 on red comet or something, and either nobody comes to the new server, or once one or two leave, they all come to the new server. People will always go where the biggest game is, and lump together. It is really frustrating, especially if you're not really into playing massive games.
The only real success we've had with this recently is chickens vs a regular game, or FFA vs team, or, sometimes, 1v1 vs team (because this only takes 2 people). This is simply a matter of playerbase. When we have enough players, we will have more than one room open at once. But when we only have enough players to fill up one room, that is all we will get.
When we are bigger, we will almost certainly split up games by elo rank. Until then, i suggest noobs play coop vs AI and chickens- some easy one-click way to set up such a game might be good.