I definitely would not add moss. Also Katherine has great tasteThanks, I tried to go with the rule of 1/3rds.
As for bracing the plastic, you could always clamp a piece of plywood to it.
Actually, I'm not such a big fan of glo-fish. However, I set the tank up for Katherine and she has free reign on what fish goes in it. This is also the reason for so many fancy goldfish. I'm not a fan of those either.
I've considering adding some moss on the driftwood but, from what I understand, it's very invasive and once you have moss, you always have moss. There are some other options though.