I found this on the net but I don't know if it is true or not...and it not really about mixing different types, It is about mixing horses from different breeders also.
"I am of the belief that the main problem of mixing seahorse of different species with similar care requirements is bacterial. Seahorses can be asymtomatic carriers of the bacteria vibrio, sometimes found in there digestive tracks. There are several different strains of vibrio carried by seahorses, which is where the problems arise.
Not all seahorses are carriers of vibrio.
A healthy seahorse who is an asymtomatic for vibrio will excrete the bacteria during the process of digestion into the water column. The bacteria will exit the seahorse through its feeces and enter the water column. This can further spread through the system if your system contains items such as live rock, live sand, copepods, and other small life forms. The bacteria can be introduced to new other seahorse specimens during ingestion, or when they swallow water that is present in the water column.
As an asymtomatic carrier of the bacteria a seahorse can live a long normal life, showing no signs of symptom or illness. Often when a carrier of the bacteria is exposed to an undue amount of stress the bacteria will produce and excrete proteins, resulting in illness of the carrying specimen. It is believed that asymtomatic carriers do have some type of antibody response classically thought to be antigens in the cell wall.
Seahorses from the same region or breeding facility can all carry the same strain of the bacteria and subsequently are fine when housed together in the same system. The problem arises when seahorses from different parts of the ocean, or different breeding facilities, who carry different strains of the bacteria or who are not carriers at all, are exposed to each other. The exposure to the live strains of vibrio will cause illness, and most often cause death.
Many breeding facilities and aquariums have experimented with different ways to vaccinate there horses against the different strains of the bacteria vibrio. Shedds's tried using cold water salmon, hoping that the crossreactivity would protect the seahorse from other strains. Seahorses Ireland claims there seahorses have been vaccinated against all strains, but they don't respond to emails. Oceanrider has received a high health facility status, which means there horses are not carriers, and safe to be kept together. They have not responded to repeat inquiries if they are safe when exposed to other horses. We would all love if any of the fore mentioned facilities responded in this forum. Remember we are your market.
It is proven that seahorses can be asymtomatic carriers for more than one strain of vibrio. This has found to be true by the necropsy's of multiple seahorses who were carrying more than one type of vibrio, but did not die from vibrio related diseases."