It isn't a surprise that your other piggies has it TBH, they like to keep such close proximity .They are prey animals so cope with stress by sticking together. Don't forget they've have several moves (breeder-stockist-you)
Fungi have an uncanny ability to survive unfortunately, but keep up with the treatment and cleaning. Ill/stressed animals are more likely to get skin conditions (most piggies 'carry' a mite population but it doesn't cause any problems unless they are ill, then it can flourish)
Are you taking them back for a check up? Ask the vet about mites treatment too (preventitive) I know some owners don't like the Xeno 450 but I've used it for years though of course, you cannot 100% say that every animal is ok with it.
I feel very and that your piggies have to go through this at a very young age when they should be settling in and getting to know you all . There was a Watchdog programme a while back about this , I suppose it depends where they're coming from. Hopefully the other pigs that were with yours will be cared for as you are doing .