A 55 Gallon aquarium is the bare minimum for a single adult Jack Dempsey. It is possible to house a pair in the 75 Gallon but please be aware, these fish can be extremely aggressive when they are put under duress and will certainly show it. I would add nothing more to the aquarium than the pair of JD's. You would be able to house a single adult JD + the convicts if you did not pair the JD, however.
You should realize that both mentioned fish are highly territorial and if not given ample room to establish their own territory, chaos will erupt in the aquarium. 75 Gallons for a pair of adult JD's would be "on the edge" in regards to territory as they can grow up to 10", they are extremely large, territorial, and aggressive. Jack Dempseys did not acquire their name from being cuddly toys, they rightfully earned it. The same rule applies to convicts.
Both are amazing fish to have though!