If you have fish that are tolerant of salt and high temps, then I would go down the route of just raising the temp to 85, and adding salt instead of treatmeant, as it will become useless if you have carbon
Catfish are scaleless so you'll have to use a half dose of salt rather than a full dose and you should definetely add it slowly to the water over a period of hours. Tetras don't really like salt either.
whitespot treatment will NOT kill the filter bacteria. But you will need to remove any carbon pads or media before using it.
Should be fine with those fish, but always read the instructions carefully to see if any fish should not be exposed to it.
in addition to the treatment, increasing the tank temperature to 30c (or has high as your heater will allow) will speed up the life-cycle of the parasite and help to ensure that the treatment erradicates them all during the course.