How much fish can I have in my tank?

Hi. I have been looking at putting a snowflake moray in my tank but I was a bit worried about over crowding my tank and I looked it up on good and they are saying about a rule about how much surface area of water to fish. What are your opinions? My tank is 250 liter so about 50 Gallon, it is more square than rectangle with depth of 17'' hight of 21'' and length of 29'' and a sump of 24'' length 10'' depth and 12'' high so adding about another 100 liters. In my tank the only fish is a lion fish what I have now seen may sadly be a little big for the tank, would doing regular water changes make it OK to keep the lionfish in there? And what about the lionfish and the snowflake moray? I also have a couple of sea urchins and a brittle star if this could change anything. I have a protein skimmer bigger than is needed for my tank and I have just purchased a media reactor what will be coming tomorrow and carbon dose with a little vodka every night to help keep the water clean. I also do quite big water changes mostly every week some times every two weeks. Any advice would be grate could potentially be picking up the eel tomorrow!