I just am not sure you realize how much information you are asking for. Books on cichlids are 100's of pages and they typically only focus on one type. For example, there are individual books dedicated to each of the rift lakes in Africa and that is only a portion of the overall cichlids.
To answer your question. Some can be mixed, some can't. Malawian cichlids and discus can never be mixed. Even just within Malawian cichlids some can and can't be mixed.
Most of the very brightly colored cichlids are from Lake Malawi in Africa. You could indeed fill up a 6x3' tank with malawian cichlids. These types of cichlids are usually overstocked to reduce aggression. I personally think that would be a total waste of a big tank. For the most part Malawian cichlids are brightly colored but extremely boring. They mindlessly chase without end.
I would really recommend doing a lot more research before you make a decision because cichlid compatibility is extremely complicated. You may find that once your tank is partially stocked you can no longer have the fish you really end up wanting.