TSC has 3/8" and 1/2" pipe thread flow restrictors. You may need adapters depending on the fittings and size of your hoses. The restrictors are about $3-4 and you only need one. For small cylinders, they slow the flow down nicely. There are other fancier and more expensive restrictors.
For 2100 rpm at 15 gpm, you will need a motor with 1.65 cu inches of displacement. At 18 gpm, you will need a motor with 1.98 cu inches displacement. I don't see any need to run at WOT. Shear pin may be nice, but a small broom shouldn't have much force/resistance.