Regarding nitrile - two values that I have are 0.005 mm (or 5 micron) for drawn rubber tubing and 0.05 mm (or 50 micron) for rubber fire hose. You have not specified the fabrication method for the piping but hopefully these two values will give you a ballpark. You should calculate the pressure drop for each of these two values, and probably others like 0.1 mm to see what the impact of the roughness is. Depending on the Reynolds number, you will find that the impact of the roughness on the pressure drop can be very small (if the flow is in the "smooth" regime).
According to the manufacturer of the Nitrile hose I am concerned with, it is "manufactured according to 3A standards which allow 1/32"of a pocket/ridge. The hose is also manufactured on a chrome mandrel, which produces the smoothest possible tube."
If you are talking of "ridges" then it sounds like you have a corrugated hose. This is typical of wire reinforced hose. If this is the case then using roughness values in the Darcy-Weisbach equation is the wrong method to use. Manufacturers usually issue graphs or tables of pressure drop data for their particular corrugated hose, and these would be more accurate than using the roughness.
