there are more kinds of mushroom than just ricordea. I would go with rhodactis or something. Duncans are (argued) to be non-photosynthetic. Some people think they prefer some light.. not just a dark tank, but yes, they are a low light coral, and they prefer lower flow so I dont see why you couldnt keep a frag and try it out. Just keep in mind that the smaller the tank, the smaller the margin of error so you have to pay close attention to your levels and water topoffs/changes. Theres no reason you cant keep some corals in there... maybe some people will post pics of thier 10g!
Jess
ps, where in fl are you (Im not familiar with the city). you could probably get some ppl to give you some small frags to try out! Im in s.florida and I know I have some small stuff I would part with.
Oh.. get a few featherdusters too! they fill in the 'holes'