Thank you for all of your replies so far. I love the bigger tanks, I just don't think I have the room for it. I'll look into it a little more and see if I can do some rearranging. Who needs files in an office when you can have an aquarium, right?
The 2.5 gallon I have set up right now seems to be doing really well. The parameters all look really good. I have some polyps, zooanthids, small kenya tree, and coralline algae growing on the rocks. The main problem is the lights are half blocked by a built in filter and there is not enough water flow or filtration to make the corals really thrive. I'd like to get everything moved into a tank actually made for a reef. I still think I need to keep it at 12 gallons or less due to size restrictions. I also do not plan on putting any fish in the tank, maybe just a couple inverts.
Thanks to everyone!