With the majority of tetras, the more in a school the more comfortable they feel. This is because of the genetic predisposition of "safety in numbers"- the more fish in a school, the less likely will be a meal of a larger fish.
When comfortable, tetras will display natural behaviors, which includes schooling.
But, there are other reasons a tetra group wont school that doesn't have anything to do with numbers. Black-water species prefer dimly lit aquariums as they come from densely covered streams with tea stained waters.
I have green neon's (Paracheeirodon similans
) in my discus tank, these fish do not like bright light and retreat to foliage when have my light at its brightest.
Here is a quote from Simply fish that talks about the areas they are endemic to:
Unlike its congeners P. innesi and P. axelrodi this species is an exclusive inhabitant of blackwater environments and tends to be found only in the upper, terra firme, reaches of tributary drainages.
Such habitats typically contain slow to moderately-flowing water with thick, often overhanging, riparian vegetation and sandy substrates covered in fallen branches, tree roots and leaf litter.
The water is typically acidic, of negligible carbonate hardness and conductivity and stained brownish due to the presence of humic substances released by decomposing organic matter.
Although it occurs over a similar range to P. axelrodi the two are not found together and P. simulans also tends to occur in warmer waters.
Paracheirodon simulans ? Green Neon Tetra (Hyphessobrycon simulans) ? Seriously Fish
Need to know about the fish you are considering before can make decisions on stocking. Simply fish is an excellent online resource to do this. Look up the Embers habitat information- see if they can take more light then the green neons. This will make just as much impact in your tank as schooling size
I see no problem with adding 8 pygmy corydoras. But, they need to be in bigger groups I would do no less than 8. You will need to do some weekly water changes- the volume will depend on water parameter tests. If you foresee issues with this obligation, then I would forego the corydoras.