Firstly 75 gal means absolutely nothing to me, it is the footprint of the tank that determines the amount of fish it can hold. Having said that I would suggest that the species to look for, for a tank with those temperatures would be C. sterbai; C. gossei; C adolfoi or C. sussei, these species do prefer warmer water and a group of 6 would be about right. I would not however advise adding Corys to your tank to just clean up after the Discus, they do have a different dietary requirement than Discus and high protein Discus food would be detrimental the the Corys health.
Ian