Would not do it long term with IT. I've had big IT's and they were mean to any cichlid I put with them. These were big cichlids that had girth to them. I would imagine a big IT would try to put the discus through the glass some day. Discus, I don't think would be comfortable eating with IT's in a tank and would prob stunt if the discus were small and needing frequent daily feeding to grow properly. If I was determined to try to make it work for a while, I would get 6inch+ discus with 1 to 2inch IT's and cross my fingers for long term coexistence. If your tank was so overwhelmingly large where the discus could get lost, the %'s of success i'm sure would rise. I would keep the temp around 82 or 83 just to make sure the dat doesn't sweat it out if attemped
Never had a big NTT, so not sure of their attitudes towards cichlids esp a sensitive one