Personally I keep no fish with my seahorses. I have reef tanks for fish and seahorse tanks for seahorses. The biggest reason is because reef tank conditions are not proper seahorses conditions. For example, temps should be 74F or lower for seahorses with low/moderate flow. Corals generally prefer warmer temps and moderate/high flow.
However, if you choose to mix the two, the seahorses should go in first, otherwise the fish will already have established territories and may cause problems.
Best of luck to you.
Tom