Depending on the size of the cube and the size of the seahorses, I'd feed half a cube of mysis twice a day for smaller seahorses, and a full cube twice a day for larger seahorses. And once or twice a week I'd replace one of the mysis feedings with a brine feeding, but otherwise not feed the brine since it is pretty much just vitamins and not nutrition (protein, fat).
I also wouldn't add pipefish since usually mixing seahorses and pipefish results in the seahorses dying, the pipefish dying, or both the seahorses and the pipefish dying. Syngnathids are very sensitive to bacterial diseases carried by other syngnathids, so it is even dangerous to mix different species of seahorses. The seahorses likely are carrying bacteria that could kill the pipes and the pipes will likely be carrying bacteria and parasites that could kill the seahorses.
