Temperature is your problem here. All varieties (which are all selectively bred from the natural species Mikrogeophagus ramirezi) must have warmer water than the majority of "tropical" fish; 82-86F/28-30C is the range for the blue rams.
Unfortunately, there are no species of Corydoras that should be maintained in this high a temperature long-term. Sporadic summer heat waves are usually not problematic, but continual high temperatures (above 80F/27C) are not advisable.
Temperature is crucial to fish because they are ectotherms, and a degree or two can make a big difference. Temperature drives the metabolism, and the higher it is (higher above "normal" for the species) the more energy it takes out of the fish, and they weaken which exposes them to disease and other issues unnecessarily.
Th Bolivian Ram, Mikrogeophagus altispinosus does not share its cousin's need for high temperatures, so it could be worth considering. Not as colourful, perhaps--depending upon how you view "colourful"--but compatible with Corydoras.
As you mention a "pair," something that applies to both species: they must select their mate (a male/female pair) and bond, or dissention will likely result. Two males will fight unless in a very large tank, but even then it can happen. The weaker will not last long. Two females may have better manners, though hard to say. Like all cichlids, males are territorial and will establish their space which can often be the entire tank. My male Bolivian was in a 5-foot tank and he owned the entire space, and every other fish (60 cories, over a hundred characins) knew it.