If you have any live rock in the refugium or any part of the system it is connected to, then you will have mysid shrimp and various types of pods
already in existence.
They will grow in numbers until limited naturally by the tank size and food availability and suitability.
Of course, when you put horses in there, they will decimate the main population and it will not be able to maintain the numbers the same as the pre horse days. You will have to augment the natural foods with either gut loaded live brine shrimp, or, feed frozen mysis to them, assuming you will only buy captive bred horses. (farm raised or pen raised, while they will eat frozen food, ARE NOT CAPTIVE BRED, and have the potential to carry any pathogens normally found in the area of the ocean waters where they are raised)