I have about five different tanganyikan species together. However I do have them in a 6ft tank.
I have worked out it is about cospecific aggression and having enough of each species to combat that, where you put your hidding areas and staying away from the real bullies of the cichlid world. I also started off having mine as fry so they all grew up together.
In my tank I have a mixture of rock and wood with narrrow leafed, windlow and normal java fern, plus anubias species (A-town, I hope you read this and remember to find me more of the different anubias species ). Quite a few of the cichlids have made the plants home and the smaller ones hid in it for protection.
My 5 shellies which also live with the larger cichlids and have about 70 shells to choose from, which have been spread thoughout the tank to help with aggression.
As ryan mentioned above, before you buy, do a little research on the species you want and see if they are work together, or work out ways to help them settle in with each other.