For now you can still see your casual rank on the ladders, but looking forward to steam release I don't see why we should show casual ranks at all. The matchmaker makes a nice distinction, splitting the games where I mess around with coms, rushes and funny game modes from the ones where I'm actually trying to win. Why would you create a ladder for the former?
In my elo analysis I've seen that the current big team game outcomes are mostly random, which is why we would first have to establish a competitive environment before there's any point on ranking players in this game mode.
A possibility would be clanwars, but those already exist, are better for ranking clans than players and are way too rare to be considered as competitive teams right now.
So it boils down to the old conclusion again that, without a better rating system, we are simply unable to give out reasonable team ranks.
As for small teams, most of the active players have already switched to using the matchmaker for those games. I wouldn't even want those 1100+1800 vs 1600+1600 that are played outside of MM while waiting for more players to count.
For the players that don't play matchmaking and are thus depicted with a wrong rank in the forums/game it might be a good idea to display them as unranked or with their casual rank ifneedbe. We could use the grey rank icons for that and shift the other icons down.