1. It is generally a good practice to purchase the largest aquarium that you can actually afford. A larger aquarium will be less susceptible to quick changes in the chemistry, and will offer much more stability.
2. Cichlids tend to be very intelligent fish. You should do your best to interact with them as some species will respond positively to you arriving home. These fish are very active and if you interact with them, they can develop much better.
3. It is not considered a good idea to keep aggressive fish with fish of other temperaments. You should keep peaceful fish in an aquarium with other peaceful fish to avoid any sort of fighting between the fish. Aggressive fish should only be kept with other aggressive fish.