I found this article from a British website about skin sores and it also says it can be either flea related or mites. Judging from the answer, it's pretty hard to pinpoint what a cat can be allergic too, but it seems as it's more than one cat they are definately having an allergic reaction to something in the house?

The article is here:
http://www.petplanet.co.uk/vet_answe...?thread_id=509


I hope you get to the bottom of what's irritating the kitties, bless them, they're in my thoughts.