There are many different problems associated with mixing species. Sometimes it is behavioral with the larger speciments attacking the smaller seahorses, issues occur with hybridization with the fry being sterile, but the main problem associated with mixing different species is bactrial. Both myco and vibrio can come in to play. Myco there is no cure for, nothing to do about and it is on the more rare side. Vibrio is very common.
IMHO if you want to mix two different species your best bet is to get them from a breeding facility where they have already been exposed to each other. Some facilities do not seperate the tanks between different species, Oceanrider is like this.
IMHO and the opinion from a few others it is my very unproven theory that vibrio diseases are most often a secondary infection.
We put so many things in our system without thought that have a very high potential to carry parasites. Snails for instance. How many people here QT there snails? Parasites living on the shells of snails are quite common.
IMO once a seahorse is bothered by certain stressors (i.e parasites, malnutrition, temperature levels, to great of current, lack of 02, presence of ammonia, etc), it falls into a weakend state letting the bacteria's take over.
To expand on the vibrio concept.
It is one nasty bugger.
There is currently believed to be 34 different strains of the bacteria that affect fish. Scientists have cultured several different strains in different seahorses. Seahorses of the same species have often been cultured for different strains of the bacteria. Some seahorses have even been cultured carrying more than one strain of vibrio.
These strains live in seahorses in an asymptomatic carrier state. That means a seahorse can live it's full life carrying the bacteria, never show any sign of disease and die of old age. It also means this same seahorse never having shown even the slightest symptom of a vibrio related illness, has the potential to infect your entire system with the bacteria.
The bacteria has the ability to live and be carried by different crusteceans and annelids. Dr Ron Shimek has found that in a square foot of sand in a home aquarium there can be between 20,000 and 80,000 different organisms. This is why the bacteria has the potential to survive in a system long after the horses are gone. There have been reports of hobbyist keeping a system completely fallow for a year after a vibrio outbreak, but when intorducing new syngnathids, the out break of the orginal strain occurs.
Vibrio bacterias are nuetralized by UV filters at exposure times from 10 to 70 seconds. After 70 seconds of exposure the bacteria has the ability to mutate to form a UV resistan strain.
Vibrio has the ability to live at temperature close to zero, although most strains that commonly affect syngnathids become less virtulent at temps lower then 69F. That is why the new trend in temperature recommendations has been on the lower side of the spectrum. The bacteria reproduces faster and is more aggresive at higher temps.
Scary huh. Here is the scarier part, it's everywhere. Vibrio is commonly found in the ocean in swarms of zooplankton. Over 40 different pathogens have been detected on artemia shells including vibrio.
Vibrio is not passed down genetically even through the gestation period, so exposure has to come after birth. There is some dispute on the exact percentage of seahorses carrying vibrio, while one scientist specializing in seahorses has the number around 60%, other scientist including Shedd's puts the number at 100%.
So if it is every where then why worry?
It is my opinion that with proper husbandry techniques that the effects of vibrio can be greatly negated in our home systems.
Mixing different known carriers is always dangerous, but with a little extra care you can do it. The major problems usually arise when mixing two different speciments with no previous resistance to the others strain of vibrio. This is the main problem with mixing species.
While some strains of vibrio are extremely aggressive and have not been succsessfully treated, most strains are managable with antibiotics. The most common problem related to treating vibrio infections in seahorses is under dosage. While it can temporarily cure a seahorse of the outbreak, it is likely to make the bacteria anti biotic resistant, which will make it nearly impossible to treat in the future for that horse, or other syngnathids affected by that new strain of bacteria. There is a product developed for seahorses called Neo3 that is designed to treat vibrio infections with the proper dosage.
If you still want to mix. I believe this to be the safest way.
Buy the seahorses and QT them seperatley, unless they come from a breeder where they have previous exposure to each other. Do the appropriate QT proceedures for the circumstance (different for WC, Aquacultured CB, and Maricultured CB). During the QT process I would recommend supplementing there food with an immunostimulant such as Beta Glucan.
I would then move the seahorses into a second tank (not the final display, not one of the tanks they were QT'd in) where they would meet each other. This tank should be bare bottom, void of liverock, livesand, just some hitches, preferably plastic. Filtration should be accomplished mechanically and a UV sterilizer should be used. Tank temp should be kept close to 68F, 20C temps given are for tropical seahorses).Water changes are going to be essential. Continue to supplement foods with an immunostimulant.
During this time period keep a very close eye for any sign of bacterial disease. If one presents, remove the infected horse to a hospital tank and treat immediatley with Neo3.
If after 6 weeks all is well, then they are ready for the display tank. I would continue to keep the temperature in the lower spectrum and to run a UV sterilizer.
Sorry for the short post, I'm kinda short on time today.

If you have any questions I can explain in more detail.
Kevin.