OK, so if I am reading this correctly, you added the new guppies to an established tank that already had fish in it and it is just the new guppies that are dying and not the originals?
What is a possibility, is that your originals have some sort of "bug" that doesn't bother them but the new fish have not encountered and so don't have an immunity to. Once the new fish started getting sick, it increased the number of "bugs" so now the originals are having to fight the larger number.
People always say you should quarantine new fish for 4 - 6 weeks to check they don't bring anything with them but that doesn't help if it is a case of something the fish have not built up resistance to. I had a similar problem once, many years go now, and it was not the fault of the fish shop. From memory, we just had to wait to see which ones survived. I don't think adding further meds to the water will help. Survival of the fittest.
Do the sick fish have stringy poo? That can be a sign of internal parasites.
Next time, quarantine new fish to check they are fine before adding them to the established tank. If they were sick to start with, they would show signs in the quarantine period. If they don't show signs of sickness until a week or more after being added to another tank with current inhabitants, I would look at something in that tank that is the problem. Not necessarily poor water quality, as it sounds like everything is good there, but something new to that particular batch of fish.
Someone else may have other suggestions.