If you are looking to have a cichlid tank you're going to want to keep them mostly the same size because cichlids are territorial fish and can be pretty aggressive. You will also want to include many rocks and different structures because cichlids like to have their own territory. I would suggest to start looking with South American Cichlids versus African Cichlids because South American Cichlids tend to be less aggressive, so you might get away with various sizes.
Good luck, I hope all goes well.
Just Keep Swimming, Swimming, Swimming
-Dory from Finding Nemo