To answer your last questions, personally in brackish setups that I've done I'm not a stickler on the salinity, it varies from time to time and I don't do exact #s. I use a basic pet store hydrometer since the exact numbers aren't crucial. Converting over to brackish is typically a very slow gradual process, often spanning months, so even if there was a different strain of bacteria involved in processing nitrogenous waste, it'd have plenty of time to colonize.