Spiny eels are the only freshwater eels. Snowflakes can live in freshwater for a short time but they will die if not transferred to a salt water tank. There are plenty of eels that are small enough for your tank. Peacock eels get about 18" there is starry night eel which stay smaller than that. Research spiny eels. Any eel will eat any fish it can get in its mouth. so keeping them with small community fish is a good way to lose fish. They are also nocturnal coming rarely in the day and will even hide under sand or gravel. As for the stingray 100 gallons is way to small. When people say 12" for a stingray they mean a 12 inch disc not total length. Cichlids are they way to go. If you ask around the Cichlid discussion plenty of people will help you make the right decisions. They are aggressive but no so much to their own kind. Aggression is not something to make you turn away from a fish. Your going to have unique problems here and there no matter what you decide, but it's nothing to be afraid of.