I agree generally that tankmates tend to be a bad idea with seahorses. I have only ever kept them on their own or with pipefishes (although I don't even recommend this - because pipefish are usually WC and its best to get CB seahorses, so you run into cross contamination of diseases).
That being said, a dragonet/scoot blenny is probably one of the best choices as far as fish go, since they are usually very passive and shy.
I have kept mushroom corals and gorgonians with ponies... but I would suggest planning an aquarium firstly around the seahorses & their hitches. Get that tank up and running, get your ponies happy & healthy, and worry about other things later.
Really you should have 2 tanks, one for ponies and one for corals & fish.
(Or do like us when we had 1 seahorse, 2 reef, 1 fish-only, 1 coldwater, and 1 african cichlid
)
-Diana