The size of the group is actually a factor in determining the lifespan of the fish. It relates to stress [another prod at that article I am working on in between my posts
]. This applies to all shoaling fish. They simply have an inherent need to be in a group of their own species, and if they are not, they exhibit heightened stress that becomes chronic, and this always means a shorter lifespan even if no other health issues occur (which they usually do). Science has now (for the first time) carried out some research that indicates the number 5 or 6 as being the crucial minimum for many species, and these studies looked at increased aggression resulting from small groups. One thing is certain, the more the merrier because the healthier--i.e., less stress--the fish will be.
Corys seem to do best in groups of 5 per species [can't remember which ichthyologist noted this, could have been Dr. David Sands], but unlike characins, mixed species do seem to play into this, as redchigh mentioned. Which is why I usually suggest 3 minimum of each species when there are 2 or more species. Though 5 or more of each is not going to hurt, if space allows. I have had many years of observation with multiple cory species. At one time I had some 60-70 corys of a dozen or more species in my 90g and 115g tanks. Presently I have about 30 from 12 species in my 115g. And my cory fry that appear now and then tell me they must be satisfied.