If it's your first tank, I would recommend using the 10g and start out with a pair of darts. Get some experience. While you are enjoying them, move onto building the 55. The key to keeping humidity in the tank is sealing the top. Darts do not need a lot of ventilation. In fact, opening the tank for feeding or misting each day will provide enough air circulation for them. The monsoons, in my experience, are hit and miss. Some people have success with them, others, like myself, haven't. I went through 3 in a year. I would recommend a mistking system. It's expandable, so you'll be able to add the 55 to the system later on. And the mistking is comparably priced, about $100 for a starter kit.
If you haven't already, read over my construction thread. It's a stickie at the too of the page. I have covered the beginning basic elements so far, and will be adding more to it soon. You may find some of it helpful.
Education is the most powerful weapon we can use to change the world ~ Nelson Mandela