Diarrhea IS a "bad sign". It can kill animals (including humans) by dehydration if it's severe...so yes, I'd take them to the vet asap with fresh stool samples.
Very suspicious too that all your snakes from that source have it. Longer quarantine for sure.....I hope they make it. BTW, going into shed is what snakes
do when they are sick or injured, it's an attempt by their bodies to repair something. So while all healthy snakes do shed & it can be perfectly normal, the
fact that these all have diarrhea and are going into shed is significant. They are sick.