The bin your pots are in should be light proof because light kills roots. Paint the outside of the bin with black paint and put some mylar over the lid to block the light. You might want to paint the lid black too.

As far as adding potted plants, I would advise against it. If anything, you could probably add another hydro system in there similar to the one you have. Also, you are going to run into some problems when the plants get larger as your pots are only a couple inches apart. Lowrider is a bushy plant and the 400W light is going to have trouble penetrating through the leaves of 6 plants if they are that close together. The lower areas of the plants will suffer.

Also, 6 plants is plenty for the 400W light. I wouldn't add any more.