VERY FEW people have succeeded keeping pipefish with seahorses long term.
Almost all pipefish are wild caught and as such, carry pathogens that the seahorses almost always fall victim to and die.
As mentioned, the minimum recommended size for seahorses that we get here in Canada would be 29/30g with an extra 15g for each additional pair of seahorses.
There is a lot to research before setting up for seahorse keeping if you want the best chance of success. Seahorse keeping even doing everything "by the book" is no guarantee of success.
As mentioned previously, it's best to set up a species only tank, and nothing but seahorses until you've accomplish being able to keep them for a year or so before adding tankmates.
Sometimes adding other tankmates leads to the same problem as adding pipefish, in that it introduces pathogens that the seahorses haven't grown up with and are unable to handle.
The only true captive bred seahorses coming into Canada at the moment are from Aquamarine International (reidi and comes), carried by several stores out west I believe. Size and quality of these have diminished recently though.
The others are erectus from seahorsecorral.com in Florida and available only from Sea U Marine in Markham Ontario. (He ships) These are the best conditioned seahorses I've ever seen coming into Canada.
There are a bunch of links at the bottom of "
My Thoughts on Seahorse Keeping" written by experienced keepers, mostly from the "org" and one article by Dan Underwood of seahorsesource.com. You will probably have to become a member of seahorse.org to read the links that are to that site, but that is a plus thing to do anyway if you are going to keep seahorses.