First you need to use a wide spectrum antibiotic like Furan-2. If you raise your temps it will hinder parasites but bacteria will explode in growth with higher temps. Salt added will not hurt but generally will not help with parasites or bacteria once they are established. Epsom salts can...
Feed as much as possible. Vac all uneaten food and w\c at least every other day. I feed ground beefheart mixed with spirulina and color enhancing flake. This food is messy vac is necessary.
Feed them a bunch before you leave. Then do a big W/C. Dont bother with an auto feeder. Then come home, feed again and W/C. You can easily get away with not feeding for 2-3 days.
If the Discus are healthy adults and placed into the 135 with a group like 5 they should be fine. The temps for adult Discus should be a minimum of 82 your tank mates should be made accordingly. I dont see a problem with any of those tankmates for healthy Discus except I dont know their temp...
Yeah I have used it a number of times. It smells funny for amin or 2 and makes the surface a little foamy but not too hard on the fish. I have found water changes, salt, temp increases will not stop an internal parasite infection. It will definately help prevent it but not a preexisting case...
Non-Wild Discus
Snakeskin
Melon
Pigeon Blood
Red
Cobalt
Blue Diamond
Albino (occur intentionally in the hobby)
Any Discus variant indicated with a #
Many more......
Just cause the poo is black does not mean a lack of parasites. I learned that first hand. If you are worried you could try a single dose of prazi pro. Your growth probs are most likely related to poor water and stress from where you got the fish.