82 - 84F is fine for discus. Higher temps (86 - 88) work well to speed up their metabolism and encourage them to eat, but you don't need to keep them that warm all the time. However, extended periods below 82F will certainly have an impact. Try to make the sure the temperature doesn't fall below 82F for them.
Due to the hierarchical social structure of discus, it's best to keep them in groups of at least four but preferably six to eight. They form a social structure with a dominant fish at the top and a submissive fish at the bottom. When you only have two or three discus, all the conspecific aggression gets directed to one or two fish. Without other fish to spread this aggression out amongst, the one or two weaker fish will often remain in a constant state of stress which will cause them to hide, quit eating, and usually leads to secondary issues like hexamita because the immune system is weakened. In most instances with people who keep two discus, one will stress the other until it wastes away and dies, and then you're left with one discus.
I have kept discus for years and I am not afraid to mix them with unusual tankmates, as long as the water parameters are suitable for the tankmates I choose. What you do have to understand is that even if another cichlid is not directly attacking/harassing your discus, their aggression and behavior in the tank in general can still stress the discus. For instance, if you keep discus in a boisterous environment with active nocturnal fish (like some species of catfish or loaches) or fast shoaling species (like silver dollars), the discus tend to be jittery and on-edge all the time. Again, this leads to a constant state of stress, which is bad for any fish, but especially discus. For this reason I would not keep them with cichlids like jewels or JDs.
Back to your issue at hand -- it seems very odd that you'd have an ich problem if the temperature was in the 80s. Are you sure it's actually ich? Can you post pictures for confirmation?