With that nice low pH, I would be going for tetra type fish. My favourite shoalers, and a bit bigger than the other tetras are the congos. Beautiful colours, and lively fish! An alternative to them would be rainbowfish.
Then what about some cories julii for the bottom, or loaches. I am choosing the julii's as they can be in somewhat warmer water, around 24 -25 degrees (sorry, I'm a metric person!).
As a centrepiece, you can take dwarf cichlids, or any other south american cichlid. You can also choose gouramis (another of my favourites), just don't take dwarfs as these are prone to diseases due to inbreeding.
ANd for the top area some killifish. I would love to have the A. australe, a bright orange fish, and not too difficult for beginners.
Sounds like my ideal tank; I am working on a 180 litr4es (47 gallon) tank at the moment.