The only realtionship they have is that they are in the same family, syngnathids. Other than that, they have no symbiotic relationship. It is possible for them to hitch on the ipes, but that is just because the pipe was in the right place to be hitched onto.
AS for keeping them together, it can be done but it is not reccomended. The WC pipes are going to be harboring all kinds of parasites and pathogens that a CB horse has never ben exposed to and therefore has no biological defenses against them. IF you put the pipes through a lengthy QT and treat them as you would a WC seahorse with regards to deworming and prophylactic treatments, it will lessen the chances but not totally remove the risks involved.
Seahorses or pipefish with appendages?
If pipefish, the only ones I know of are the ghost pipefish and they have never been kept successfully in an aquarium. I know of a few well expereinced keepers that have not ben able to sustain them for more than a few months. Other than that there are no other pipefish with appendages that I have ever come across.