I don't intend on continously adding salt to the tank, just only for the first week or so until my fish settle in. In the past when I add fish to a new tank ocassionally one or two fish out of the lot would develop ick, I found that by adding a bit of aquarium salt to the tank it reduces the severity. Also I'm sure that most Rainbowfish including my species don't require brackish conditions. That is to say I don't believe everything the LFS guy says any way, adding the salt was more to help my fish settle into the tank.
To answer the questions, I've got a pair of M. trifasciata and a bristlenose at the moment but will be adding prehaps 4 more in within the next few weeks. Also its aquarium salt that I'm using.