Some of the cast reappear in other Chicago shows as different characters for example Charlie Babbo (Ben Darrden) plays a little boy who gets badly injured during a car accident in Chicago Med (2015). All though these are two different characters they are both called Ben.
Most departments, Chicago FD included, do not allow firefighters on duty to grow out facial hair on the cheeks or on the lower lip or chin as Lt. Severide often does. It interferes with achieving a tight seal on the SCBA (air) mask as the seal gasket on the rests against those parts of the face. Firefighters with too much facial hair will also fail the mask fit test for the same reason. Mustaches (as often seen on Chief Boden, Mouch, Otis and some others) are not technically an issue, but some departments may have internal regulations that restrict them.