I'd leave the dempsey out... with a max size of 10-12 inches, 2 cons would be hard pressed to take a fish that size on... they'd get stressed.
A 75g is ALOT of space, convicts use very little compared to alot of other cichlids. I've had breeding pairs of cons in a 35 community withouyt qualms... the other fish just gave them some space and that was it.
Provided the territories are defined, a breeding apir of jewels, a breeding pair of cons, and a breeding pair of firemouths can all live in a 75 with some targets. For example, I'd put a rock pile w/ cave in 1 corner, a rock pile with cave in another corner, and a bigger rockpile with 2 caves in the centre. Between each cave, I'd use large obstructions to block the views of the fish from other pairs; large pieces of driftwood with plants attached do nicely. also, if you'd like, to even the tank out, on each corner cave/pile you could stack it reasonably high and attach some plants so the corners don't look so low in comparison to the obsrtuctions.
Yes, bottom dwellers can be kept with breeding pairs of fish privided they are tough and stay well clear of the pair's territory. Khuli loaches are like worms, they'd get slaughtered after 2 bites. However, I'd rather not use any as at night, when the parents might be taking a quick rest, they'll eat the eggs and get killed by the pair. Many bottom dwellers are persistant and keep going back for eggs, each time taking a beating, until eventually you're left with no eggs and a beat to near death bottom dweller.
For targets/dithers, I'd use rainbows, as they are more colourful, won't eat plants, and aren't as big as silver dollars. With cichlids, I wouldn't use more than 4-5 silver dollars (and that's pushing it), whereas if you go rainbows, 8-9 isn't unreasonable and leaves space for a little something extra (not another cichlid though).