Agree with what has been posted. The space limitations of a 20-25g tank limit the type of cichlids. As will your water parameters which are not mentioned.
Hard water will be good for the shell dwellers, and they are small.
Soft water, or medium hard, will suit one of the South American dwarf cichlid species, such as the cockatoo you mention. A pair in a well-planted tank, with some corys and some upper fish would be nice. We have fish profiles, second tab from the left in the blue bar across the top, or if the name is exactly the same in posts it will be shaded and you can click on it to see that species' profile. The Cockatoo Dwarf Cichlid
is in our profiles; as it mentions there, a male with 2-3 females in a 24-inch tank (your 20-25g) is fine.
An upper "dither" fish is a good idea. The Flame Tetra
would work, a group of 5-6 max in a 20g, in which case I would limit the cichlids to 3 max.