Bottomline is your tank is not fully cycled and that cause your fish to die due to ammonia/nitrite poisoning...
Damsel are notorious for their aggressiveness and territorial behavior, so best to avoid them if you want to keep other fishes...
Those are majano anemone, pretty harmless but can be pest if it start shifting and sting all your corals... Not much effect on fishes, their sting not that potent compared to other anemone...