You can pull off a salt bath with anyone, yes, but you have to vary the concentration. I'm not sure who you're working on, but catfish like ottos, plecos, and cory cats, along with similarly scaleless fish like loaches, have a quite low tolerance for salt; although they'll probably stand up to 1 tablespoon per gallon, you may want to step back some. Tetras aren't fond of it, but will tolerate it much better than catfish. Cichlids that like hard water generally have a higher salt tolerance, while those that like soft water like Angelfish are lower. Goldfish, bettas, danios, and other cyprinids have exteremely high tolerances. I've given my goldfish a 24 teaspoon/gallon salt bath before.