It might be a firm requirement but it's not an SEC requirement. Either that or there are quite a few firms that are breaking the law. P.S. Grammar check - is that supposed to be "an SEC" or "a SEC?"
It depends on how the firm is registered. With some firms you MUST have the series 55 according to the SEC. With others you don't. I can remember the exact difference in registeration and standing but I am sure someone will point in out. (e.g. with Schonfeld you don't need it)