If you tested the water after cycling and "it is all correct", why are there ammonia and nitrite there? Is it nitrite which is at 0.1ppm, and not nitrate? Just the 0.1ppm of each could have easily been enough to set off the infection in guppies. Dose treatment, water change, dose treatment, waterchange is all you can do now.. it might be best to get an anti-bacterial medication.