If you choose species that dont grow larger than 4" in size then the aquarium is nearly 5 times the length and width of the fish which i would say is fine and especially with the 5.5' height as well. How many people have a aquarium 5 times the length of the fish in the length and width of the aquarium? if that was the case people with arowanas would need a 20' fish tank lol.
People set africans up in 20g tanks lol so i do doubt your fish wont be happy with the dimensions.
In my opinion i wouldn't stock it with shell dwellers as in a 5.5' high aquarium how much are you actually going to see a 1" fish?
I'm pretty certain I'm going with small mawali cichlids. If they do look stressed or grow to big I can't always move them to one of my other tanks or even set up a new one for them. I will post a picture of the tank when I bring it home from work on Tuesday.