On Jan 1, I mixed my 4 WC reidi with my 6 CB erectus. Since then, I have added a WC false vampire blenny and a WC heniochus. To date (I know it's only been 19 days) I have had no problems whatsoever, BUT I do believe mine is a special case.
My CB horses were conceived WC and born and raised in captivity, and the water from the reidi tank was used to set up the erectus tank more than once (it developed a leak) so the horses were already basically exposed to the same pathogens.
I weighed the decision very carefully, and decided to take the risk. I am examining all 10 horses every night for any small signs or symptoms of anything out of the ordinary, and happily, so far, all seems well. They dance morning and evening, all have nice fat tummies, they are eating like pigs, and they all bop all over the tank hunting. If you have the same erectus from Karen, coming up on their 1st birthday in Feb, yours should have the same immunities.
I think people as a whole baby their seahorses too much. After all, they are still just fish. I know how bad it will hurt if the entire tank got wiped out, but after weighing the pros and cons, I can live with my decision. I try to keep my horses in the best possible health, with the best possible water quality, which is why I feel I have had the success I have. If you can find pipes from a reputable dealer that you believe in, and your horses are in good health to begin with, I think you will be successful.
BTW, I do not quarantine, and have never had a problem I traced back to not doing it.