The issue with keeping stinging celled and photosithetic critters is that they should not touch each other and have room to grow and should not shade light from others. The all have defences and a strong will to survive. Many can release toxins to ward off and irritate others from invadeing and water quantity and quality becomes a factor in cleaning and diluteing the toxins. With anemones, they can move around and sting others.
Depending on tank size, filtration, species, over crowding, lighting, ect. many people have sucsess and many have problems.
Some species can mix and touch but often different species do touch and seem fine but after awhile it can take its toll.