I'm not familiar with the filter, I am saying if the filter is listed for 55-100 gallons as a stand alone, get two.
You will need to get rid of the feeders anyway because of their voracity as compared to the fancies. They will out compete them for food and space if given the time. The 4 goldies for a 55 gallon tank would be ideal at this point. If the comets were given away, weekly water changes/gravel cleanings would be the standard but the water change would need to be at 30%-50%. Water changes being on a weekly basis is not just a goldfish thing, it is an aquarium thing. This is for Freshwater unplanted tanks though, each kind of tank varies though. Planted tanks have different needs and Saltwater tanks have different needs. As far as not needing to change a SW tank for 45 days a pop, that I am not sure about. Who is giving you this advice?