If the box is green: connected to you and everyone else
If the box is yellow: connected to you but not everyone else
If the box is black or non-existent: not connected to you

Agreed.
I have played only 7 1v1 games so far (including global 1v1s and ladder 1v1s) and my experience was that I either get matched up against a regular 1v1 player then get steamrolled or I get matched up against a brand new player that doesn't even know to build a factory first or what reclaim is. I am still not yet to the point where I can determine what the best stuff to build at first and in what order for a specific map, which is a major detriment to my performance in 1v1s if it all really boils down to build order importance.

Another thing: 20x20 maps are a major pain to deal with in a 1v1 because of the sheer amount of map to keep track of. If it were a teamgame then 20x20 would be be more tolerable because you have teammates watching the other sides of the map for you which is why 1v1s on large maps are more tiring than team games on the same map.
If we are limiting the pool to 3 maps I would suggest maybe 2 5x5 maps (Theta Passage and Winter Duel seem like a good fit to me) and one 10x10 map. (Open Palms?)

If the regulars don't like the map pool they can go host their own custom games.