This is a blenny commonly called a "barnacle blenny" or sometimes referred to as sailfin blennies or tube blennies of the genus Acanthemblemaria. They don't get very big, about 2" maximum, and are carnivores that, in the wild, feed on zooplankton. They are generally peaceful to tankmates and more than one can be kept in a tank. This is evidenced by your observations though because seahorses and pipefish don't do well with aggressive tankmates. I've heard of people keeping small colonies of them.