Yes to all three of your questions - hydroids can be introduced into your tank by live rock, live sand or from 'live' water from another system.
Panacur will kill the hydroids, it will not kill the good bacteria. It WILL however, possibly kill some corals. Some snails, macroalgae, tubeworms, bristleworms, etc cannot tolerate panacur either.
If you have/are planning to keep larger seahorses like Kuda, Reidi, Erectus you do not need to worry about hydroids. They will not cause a problem for the larger seahorses.
Hydroids are only a problem for dwarf seahorses or seahorse fry.
Tom