Scallops can be very difficult but not impossible. Overall though I would say avoid them. Scallops are strict filter feeders and also require a certain size of food or they cannot catch the necessary amount of food to survive. If the tank is "too" clean there will also be a lack of natural foods in the water column. You can try liquid foods but they are rather hit and miss which means more feedings more often and possible water quality issues. They must never be moved as well. Flame scallops use small fiber/muscle strands to attach themselves to rocks or other hard surfaces and moving them will cause alot of damage.
Nudibrancs are much in the same boat. Each species will usually have a specific diet consisting of a particular animal, sponge or algae. If you cannot supply that then they will usually pass once the natural food in the tank is gone. The best one to consider would be a lettuce nudibranc. They are strict algae eaters and can often take to prepared algae foods like nori quite easily. Make sure the tank has aged some and the water parameters are quite stable as they will perish in higher nitrate waters.
Cheers
Steve
