Sorry about all the questions!
The bit about the ram and the corys not getting on in terms of temp.
I run my tank at 26c which is top end of the cory temp (although there very active and seemingly happy - they also dont seem to group together much at all (currently 4, will add more but adding a few fish at a time due to be being a new car)) but within the range of the ram according to the site i was recommended on here.
http://www.seriouslyfish.com/species/mikrogeophagus-altispinosus/
http://www.seriouslyfish.com/species/corydoras-julii/
Lee
First, in your initial post you mention rams as "electric blue." This is a variety of the natural species
Mikrogeophagus ramirezi. This fish must have warmth, minimum 80F (27C) or higher. This was behind essjay's comment on temperature differences.
Then in post #8 you change species to the Bolivian Ram,
Mikrogeophagus altisponus. This is a different species and it can manage at less warmth than its cousin, as noted in the Seriously Fish link.
With that cleared up, there is the temperature itself and why this is crucial to fish. Fish are ectothermic and the temperature of the water drives their metabolism. Even one or two degrees can be significant for the fish. The temperature range given for most species is that within which the fish should manage, but not necessarily permanently at either end. Keeping cories in warm water, the high 70's, is wearing them down slowly. You won't see this externally. There are a few species that are frequently recommended for higher temperatures, even in with discus and the common/blue ram. But authorities like Ian Fuller (he owns
Corydoras World, has authored many articles, and collected cories in SA) discount this, and recommend cories not be kept anywhere at or above this temperature. But the point is that temperature is a crucial factor in combining fish species, and mid-range is usually the intended goal.
EDIT. Just noticed another issue was raised, namely adding numbers of a species to increase the group. With shoaling fish, always add all intended fish of that species together at the same time. In all cases this allows the fish to settle in considerably faster because the more there are the less stressed ("scared") they will be and the sooner they will settle, so less chance of ich which is a result of stress. In some species this is also necessary to deal with hierarchy issues. Not relevant to cories, but it would be to angelfish, discus, many but not all of the characins and cyprinids (loaches for example establish very strong social structures). Worth keeping in mind.