Sorry, last night got a bit busier then I was expecting!
Dwarves don't mix with big seahorses because dwarves need to be in small tank for proper food density and the big seahorses just don't fit.
Ok, think of the drinking water in Mexico - makes us sick, but doesn't cause any problem for people who are raised on it. It's the same kind of thing - depending on how and where they grow up, seahorses have different tolerances to various bacteria, depending on what they have been exposed to and at what levels. Also, different species may have different sensitivities to various things. The belief is that a seahorse can be carrying a bacteria (or a parasite, etc) that does it no harm but when a seahorse with no exposure to that comes in contact with it, the new seahorse gets sick. The same thing with pipefish exposed to seahorses - they are all syngnathids and are more likely to transmit things to each other - there is little concern with exposure to inverts or other families of fish. The biggest risk is mixing wildcaught with captive bred because the range of things they have been exposed to is radically different and it's usually the captive bred seahorses that die due to the contact. I can't tell you how many people have lost their captive bred seahorses when they added a wildcaught pipefish to the tank. There are varying degrees of risk involved. Mixing species from the same breeder is lower risk as they have likely had the same exposures. Even members of the same species create some risk if they come from different sources.
That said, I have successfully kept different species together - I currently have 3 species in one tank. I also lost seven seahorses last year to a disease that could have been introduced by mixing them. There is a risk.