Cichlid species that can live together are cichlids from the same region. For example, African cichlids go well with other African cichlids, South American cichlids go well with other South American cichlids, and so on. In addition, some species such as dwarf cichlids can live together with other cichlids.
If provided with clean water and food, most cichlids can theoretically survive on their own. However, it is best to keep at least one breeding pair or more to allow them to thrive. The only cichlid that is often kept alone is the Oscar. Solitary keeping is possible, but I would recommend not keeping cichlids singly.
No, mixing African Cichlids and Oscars is not recommended for a number of reasons. They come from different continents and require different water values. They also fight to the death depending on the cichlid.
These cichlids have complex social groups and they should not live alone but in groups of at least six fish. It is possible to provide angelfish with some tankmates, but make sure the tankmates are not too small as he will eat them.
The dwarf catfish or Otto cat (Ottocinclus spp) also stays small and is a hardy fish. A group of them gets along well with small cichlids. In addition, rubber-nosed catfish (Chaetostoma spp.) generally stays under 15 cm and does not harm small, peaceful cichlids.
Some species that would do well in a peaceful tank with your angelfish include Symphysodon and Heros species. Rainbow cichlids, dwarf cichlids, and mites are commonly available species at upscale pet and aquarium stores.
Since Oscars grow to be quite large, they will not thrive in a small aquarium and do not generally get along well with other cichlids. The key to ensuring your Oscars live full and healthy lives is to create an aquarium environment that mimics their natural environment.