Of course flue plugs were used on the narrow gage. The boiler doesn’t know what gage track it’s sitting on. The frequency of their use depended on the maintenance standards of the particular railroad in question. I would be surprised if the D&RGW used them very much at all in the latter days of steam, when the railroad was financially healthy. The RGS may have had a rack full of flue plugs, and lots of practice putting them in.