With a tank like that you NEED to get some rays! I don't know too much about them either but I am going to start researching them! Sounds like a great set-up you got going!
You'd get like 30 fish for what? $30.00? Let them "cycle" cycle the tank (then you'd have 12,000 convicts (two broods per couple w/in 2 months-so my math isn't right, but neither is a 12 x5x5 tank!!!)...then when you've made up your mind on what MONSTER will go in the tank first IT WILL BE CARNAGE!!!