Nice to see you starting slow.
However, I'm curious as to how you are going to keep the mandarin alive long term. There is not enough rock to sustain any population of natural live food for it, and even if it was sufficient as in a much larger tank with a lot more rock, the seahorses when added would decimate the pod population VERY fast. Not many mandarins can be trained to eat frozen foods but the odds are better if it is one of the cultured mandarins.
As to the comment on flow, seahorses like a variety of flow, low, medium and sometimes high flow, but just don't like to be "blasted" where they might get thrown against something causing injury. Hitching in all flow areas allows them to choose the area they want at any given time.
Corals can cause some problems in that a lot of sps need very clean water and seahorse tanks are notorious for excessive nutrient in the water due to their messy eating habits. Also, stinging corals are problematic for the seahorses.
If the corals kept require intensive lighting then you may have to provide for a way to cool the water sufficient for better chance of success in keeping seahorses as the warmer the water, the lower the chance of success due to the exponential growth of bacteria as temperatures rise.
With the rockscaping, one needs to be sure a seahorse cannot get itself trapped in cracks and crevices as they hunt for live food, whether there is any there or not. I've even had them caught in decor.
Hopefully you've already planned on true captive bred seahorses as can be obtained from seahorsesource.com, pekasponies.com, or seahorsecorral.com.