I have bred bettas in a 10 gallon, 37 litre, tank that was only partly filled. The water level was at about 6 inches. You are right about needing to keep the betta sorority separate from the male but you also need to consider the need to sex and then separate the fry at a young age.
Kribs I have bred in a 29 gallon, about 90 litres, tank. I have also seen them bred in a 10 gallon with a larger grow out tank for the fry.
I have never kept apistogrammas so I can't be much help with cockatoos.
A very easy cichlid that comes out quite nice looking is a Herotilapia multispinosa, known as a rainbow cichlid. They can be bred in a 200 litre tank and the fry can be raised in that tank to about 1 1/2 inches in size which is a good size to sell them off. They love to eat plant matter so live plants don't survive well in their tank, it does seem to bring them into condition when I give them a layer of duckweed on their tank to eat. This is a picture of some of mine.
and another picture with new fry in the picture
The plants in the second picture are just silk plants. The dark lower body on the adult is a breeding coloration that is not seen unless they are actively breeding or rearing fry.