Yellow and Demasoni are the classic combination for Malawi cichlids tank setup. They are my personal favorite still. You can get away with small tank size when they are young or juvenile size. But eventually you will need a 180L or plus for group of Yellow and demasoni. For tank size, a classic 1.2M tank with dimension 1200mm L X 400mm W X 400mm H (190L) is a good start, or 1200mm L X 500mm W x 400mm H (240L) is even batter. Malawis especially Mbuna are more bottom layer fish. You need tank footprint much more than height. For fish number, it depends on your tank size. Typically you'd better to have dozens of yellows and demasonis in each group. The more you have the less aggression in the tank. For yellows, I would say any number above 6 would be good start. They are not that aggressive most of the time but only during breeding season. For demasoni... thats another story... You can have a singel fish or have a lot, say 12 - 15 to start with or even more if you can. I would strongly against any number below 10. For red color fish that will get along with Yellow and Demasoni, red/peach zebra. They are same as yellow not so aggressive as demasoni... So just get serveral to keep themselves as a company. The other option would be dragon blood but they might grow pretty big later on. But if you raise them and grow them out with yellows and dems from young they wont grow that big due to in the same small tank, get a couple if you like them. Good luck on your setup.