Capacity at the Track and field stadium is changeable - it can actually hold more than the HDC if they install all the bleachers they have. They probably won't, though, so it'll hold about 15,000 if they open the seating on both sides. For the Galaxy and Chivas reserve games, they usually use just one side of the bleachers. The tickets for the Open Cup match are just $10 and it's general admission seating, so no fancy pants arriving late to sit in their luxury box are going to get the best view. What's especially fun about the T&F is that the stands are made of aluminum, and are very noisy if you stomp or jump. They even bounce a little. Considering that this match is going to be practically the only home game the whole month for either team, I hope it'll be a good turn-out and a lot of fun. Chivas USA is taking the Open Cup seriously - trying to follow the steps of Liverpool, I guess, by salvaging the season with a good show here. That team will be hungry, but won't it be sweet for the Galaxy to crush their dreams?
I'm fairly certain the teams that got a bye until the fourth round will have to open on the road. I guess it doesn't matter. Still, though, if Charlotte had won the Galaxy would have played at Charlotte.
Actually US Soccer accepts bids from each team (in our case, Galaxy, Chivas and Charlotte) before the previous match. These smaller teams with lower expenses usually offer some advantage there. And they can draw closer to their average than MLS teams. KC is coming in and hosting the next round. Because KC has that alternate field in Overland Park, they have less expenses to take out before dividing the ticket proceeds.