Yes, sir, mushroom corals and pulse corals (Xenia spp.) are generally perfectly safe to keep with seahorses. Soft corals have very little stinging ability and generally make good choices for a modified mini reef that will include seahorses (Delbeek, Oct. 2001). This includes most mushroom anemones (corallimorpharians). However, as Charles Delbeek cautions, "One notable exception is the elephant ear mushroom anemone (Amplexidiscus fenestrafer). This animal is an active feeder on small fish and will envelope them whole with its mantle then slowly digest them by extruding its digestive filaments into the space created. No small fish are safe with these animals in the tank (Delbeek, Oct. 2001)."
The only possible problem with the pom-pom Xenia I can foresee is that it will have to become accustomed to being used as hitching posts by the seahorses from time to time. It may take in a while to adjust to occasional contact with the seahorses before the polyps open up and resume their rhythmical pulsing which is so fascinating to observe. So be sure to watch any soft corals and hermatypic corals to make sure they are not closed up for extended periods. Normally, they adjust to the seahorses’ presence and unwelcome attention after a while, and remain contracted only briefly after each contact. After repeated exposures to grasping tails, each such incident elicits a weaker response, so they tend to extend their polyps sooner and sooner after being disturbed (Delbeek, Oct. 2001).
Best of luck with your seahorse setup and hand-picked corals, Don!