Im not sure if this has been done, but an article on velvet would be helpful to me but it depends weather its helpful to a quantity of people i suppose.
you'll have to read the articleI'd like to hear your expert 55 stock.
Nitrate, even within normally safe levels, is IME associated with what I call "small fish dieoff" - where up to 50% of a shoal of new small fish die within a short time of being added to the tank.
Another cause is acclimation. Bear with me. To explain, a bag of ten ember tetras has probably built up some ammonia in transit. Now, this isn't harmful because the organic processes will have depressed the pH. But then you drip acclimate, which means that you start putting your water - which may have a higher pH - into their water. This turns the ammonia to the toxic un-ionised form and the fish suffer ammonia poisoning.
Drip acclimation is great when you know the parameters in your tank are different from those in the bag. When the difference isn't great, I'd concentrate on getting the fish out of the shipping water and into the tank as fast as possible.
Not saying this did happen, but it's a possibility. Unless you know your water is much harder or much softer than the water the fish come from, then equalise temperatures and get the fish out of the bags. Drip acclimate only when you need to for reasons of osmotic difference. Don't worry about pH either; if the hardness (and generally therefore the TDS) of the water in the bag is close to that of the tank, a fish can swim straight from pH 6.5 to pH 7.5 and not notice. A bigger change might make it feel a bit groggy (especially if it's going from alkaline to acid) for a bit but will probably do less long term harm than ammonia.
Actually, if you have a bag of fish that has been in shipment for a while, many shippers would recommend a plop and drop approach. It is not at all the same kind of water chemistry that you deal with when you bring fish home an hour after they were bagged. In the case of shipping, there is very little water for each fish in the bag, the fish has been in that small amount of water for at least a day or two and the text message you put in the original posting makes sense in that context. The shipping water is very likely to be seriously degraded by the time you get the fish and if you are not careful the CO2 that is holding down the pH will come out of solution and the water will become quite toxic. You don't want your new fish in the water when that happens.