If you have to use the crushed coral, rift lake cichlids. Some pics of fish from both lake malawi and lake taganyika can be found here:
http://badmanstropicalfish.com/afcichlids.html
There's also fish available from lake victoria (might want to google that one: "lake victoria cichlids" or similar), however I'm pretty sure lake malwai and lake taganyika cichlids are the most commonly available.
You need to pick all fish from one lake as doing otherwise could easily result in compatibility issues (although it probably could work if you put enough research into it).
I'm not the most knowledgable with these fish, so if you would like to go with them, please post a thread in the cichlid section with the title saying "newbie starting [insert the lake you picked here] cichlid tank, need help" and these questions and any other questions you can think of in the post:
1. What kind and how many fish from [inset the lake you chose here] would you recomend for my tank size (55g)?
2. What should I know about these fish?
3. Any links to some good reasing on these fish?
4. What should I be doing maintenence wise for these fish?